Base64 string to pdf file

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Base64 string to pdf file

Comments: 92 | Rating: 4.8/5 Decode Base64 to file using the free online decoder, which allows you to preview files directly in the browser, as well as download them, get the hex dump for any binary data, and get summary information about the original file. Please note that the preview is available only for textual values and known media files such as images, sounds, and videos. In any case, you can always convert Base64 to binary and download the result as file, regardless of its MIME type. If you are looking for the reverse process, check File to Base64. What are the features of the decoder After decoding the Base64 string, you'll be able to: Preview the file (for example, you will see an image or video player). Download the file by clicking on the special link. Read the information about the original file (such as MIME type, extension, size). Analyze the hex dump of the file. You're now watching this thread and will receive emails when there's activity. Click again to stop watching or visit your profile/homepage to manage your watched threads. You've stopped watching this thread and will no longer receive emails when there's activity. Click again to start watching. Hi guysI have a java rest service answering with a base64 file content (for example a pdf or an image). But the first question is: which is the best way to display the file content?Unsure on what to do, I decided to put a webkit view in my page and to execute the following code:if let decodeData: Data = Data(base64Encoded: download.fileContent!, options: NSData.Base64DecodingOptions.ignoreUnknownCharacters) { DispatchQueue.main.sync() { self.webView.load(decodeData, mimeType: "application/pdf", characterEncodingName: "utf-8", baseURL: URL(fileURLWithPath: "")) } }But the file isn't displayed in the webview and the console logs the error: "failed to find PDF header: `%PDF' not found"I don't know how to solve the problem. I'm using swift4 and xcode9.Thanks in advance Up vote post of giamma84 Down vote post of giamma84 It's pretty clear that the data you got back from the Base64 decode is not a PDF. PDF files all start with the sequence 25 50 44 46, that is, the ASCII bytes for %PDF. I recommend that you dump the data to see what you're actually getting. Share and Enjoy -- Quinn "The Eskimo!" Apple Developer Relations, Developer Technical Support, Core OS/Hardware let myEmail = "eskimo" + "1" + "@" Up vote reply of eskimo Down vote reply of eskimo I receive a base64 string pdf from a rest service. It is a binary file content encoded in java with the method "Base64.getEncoder().encodeToString(fileContent)".In javascript i convert it into a binary array and i save it to the disk. In swift4 i tried the previuous method but it seems do not work. Any idea? Up vote reply of giamma84 Down vote reply of giamma84 His idea was to dump out the data to see what you're actually getting, not what you think you're getting. Up vote reply of junkpile Down vote reply of junkpile I printed the data in base64 string and the result is exactly what i obtain on a converter in internet. But the pdf is not displayed in webkit Up vote reply of giamma84 Down vote reply of giamma84 I think at this point you have two options:1. File a bug report containing the PDF file and your code, then submit a developer tech support incident (described here: .2. Post your question somewhere like Stack Overflow where you can post more of your code, any relevant links, and more information. Up vote reply of NotMyName Down vote reply of NotMyName How do i dump data in xcode9?You can do this using NSLog: let d: Data = ... NSLog("%@", d as NSData)Share and Enjoy -- Quinn "The Eskimo!" Apple Developer Relations, Developer Technical Support, Core OS/Hardware let myEmail = "eskimo" + "1" + "@" Up vote reply of eskimo Down vote reply of eskimo I've finally found the solution. I post it there with the hope it can be usefull to someone.I've simply decoded the base64 string twice:let fromDataToString = String(data: download.fileContent!, encoding: .isoLatin1) let fromStringToData = Data(base64Encoded: fromDataToString!, options: .ignoreUnknownCharacters)Then i was able to display the file in the webkit view.Thanks to all Up vote reply of giamma84 Down vote reply of giamma84 For the record, you aren't decoding twice. In your first post, you used a method that assumed UTF8 string data. Now you are specifying Latin1 and explicitly creating a string. I would expect either encoding to represent base 64 data the same way. There is something funky about your input data. Up vote reply of john daniel Down vote reply of john daniel Based on what I read java uses latin1 for enc/decoding base64 string and probably mongodb driver does the same Up vote reply of giamma84 Down vote reply of giamma84 Base64 is ASCII compatible, so you'll get the same representation in any 8-bit encoding that starts with ASCII (and that includes both ISO Latin-1 and UTF-8). If I were in your shoes I'd spend the time getting to the bottom of this problem rather than relying on this double decode workaround.To investigate this further you could:Decode the data both waysWrite that data out to a temporary fileDump the files to see what's differentShare and Enjoy -- Quinn "The Eskimo!" Apple Developer Relations, Developer Technical Support, Core OS/Hardware let myEmail = "eskimo" + "1" + "@" Up vote reply of eskimo Down vote reply of eskimo But why? You only need base64 if your transport layer is textbased, like XML or JSON. Perhaps the problem is how you are extracting data out of that transport layer. For whatever reason, your explicit Latin1 decoding is apparently doing a better job of that. That alone should be cause for concern since any decent text-based transport layer should be using UTF-8. I strongly suggest getting to the bottom of this issue now, before you ship this to people who use really funky text encodings. Up vote reply of john daniel Down vote reply of john daniel Yeah, my solution is quite a workaround. I have to finish a work within 4 days and I'll spend a lot of time to investigate this problem. #staytunedThanks for all Up vote reply of giamma84 Down vote reply of giamma84 Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding. Base64 encoding schemes are commonly used when there is a need to encode binary data that needs to be stored and transferred over media that are designed to deal with ASCII. This is to ensure that the data remain intact without modification during transport. Base64 is commonly used in a number of applications including email via MIME, and storing complex data in XML. One common application of Base64 encoding on the web is to encode binary data so it can be included in a data: URL. In JavaScript there are two functions respectively for decoding and encoding Base64 strings: btoa(): creates a Base64-encoded ASCII string from a "string" of binary data ("btoa" should be read as "binary to ASCII"). atob(): decodes a Base64-encoded string("atob" should be read as "ASCII to binary"). The algorithm used by atob() and btoa() is specified in RFC 4648, section 4. Note that btoa() expects to be passed binary data, and will throw an exception if the given string contains any characters whose UTF-16 representation occupies more than one byte. For more details, see the documentation for btoa().Each Base64 digit represents exactly 6 bits of data. So, three 8-bits bytes of the input string/binary file (3?8 bits = 24 bits) can be represented by four 6-bit Base64 digits (4?6 = 24 bits). This means that the Base64 version of a string or file will be at least 133% the size of its source (a ~33% increase). The increase may be larger if the encoded data is small. For example, the string "a" with length === 1 gets encoded to "YQ==" with length === 4 -- a 300% increase.Since DOMStrings are 16-bit-encoded strings, in most browsers calling window.btoa on a Unicode string will cause a Character Out Of Range exception if a character exceeds the range of a 8-bit ASCII-encoded character. There are two possible methods to solve this problem: the first one is to escape the whole string and then encode it; the second one is to convert the UTF-16 DOMString to an UTF-8 array of characters and then encode it. Here are the two possible methods.function utf8_to_b64( str ) { return window.btoa(unescape(encodeURIComponent( str ))); } function b64_to_utf8( str ) { return decodeURIComponent(escape(window.atob( str ))); } utf8_to_b64(' ? la mode'); b64_to_utf8('4pyTIMOgIGxhIG1vZGU='); This solution has been proposed by Johan Sundstr?m. Another possible solution without utilizing the now deprecated 'unescape' and 'escape' functions. function b64EncodeUnicode(str) { return btoa(encodeURIComponent(str).replace(/%([0-9A-F]{2})/g, function(match, p1) { return String.fromCharCode('0x' + p1); })); } b64EncodeUnicode(' ? la mode'); Note: The following code is also useful to get an ArrayBuffer from a Base64 string and/or viceversa (see below). "use strict"; function b64ToUint6 (nChr) { return nChr > 64 && nChr < 91 ? nChr - 65 : nChr > 96 && nChr < 123 ? nChr - 71 : nChr > 47 && nChr < 58 ? nChr + 4 : nChr === 43 ? 62 : nChr === 47 ? 63 : 0; } function base64DecToArr (sBase64, nBlocksSize) { var sB64Enc = sBase64.replace(/[^A-Za-z0-9\+\/]/g, ""), nInLen = sB64Enc.length, nOutLen = nBlocksSize ? Math.ceil((nInLen * 3 + 1 >> 2) / nBlocksSize) * nBlocksSize : nInLen * 3 + 1 >> 2, taBytes = new Uint8Array(nOutLen); for (var nMod3, nMod4, nUint24 = 0, nOutIdx = 0, nInIdx = 0; nInIdx < nInLen; nInIdx++) { nMod4 = nInIdx & 3; nUint24 |= b64ToUint6(sB64Enc.charCodeAt(nInIdx)) >> (16 >>> nMod3 & 24) & 255; } nUint24 = 0; } } return taBytes; } function uint6ToB64 (nUint6) { return nUint6 < 26 ? nUint6 + 65 : nUint6 < 52 ? nUint6 + 71 : nUint6 < 62 ? nUint6 - 4 : nUint6 === 62 ? 43 : nUint6 === 63 ? 47 : 65; } function base64EncArr (aBytes) { var nMod3 = 2, sB64Enc = ""; for (var nLen = aBytes.length, nUint24 = 0, nIdx = 0; nIdx < nLen; nIdx++) { nMod3 = nIdx % 3; if (nIdx > 0 && (nIdx * 4 / 3) % 76 === 0) { sB64Enc += "\r"; } nUint24 |= aBytes[nIdx] >> nMod3 & 24); if (nMod3 === 2 || aBytes.length - nIdx === 1) { sB64Enc += String.fromCharCode(uint6ToB64(nUint24 >>> 18 & 63), uint6ToB64(nUint24 >>> 12 & 63), uint6ToB64(nUint24 >>> 6 & 63), uint6ToB64(nUint24 & 63)); nUint24 = 0; } } return sB64Enc.substr(0, sB64Enc.length - 2 + nMod3) + (nMod3 === 2 ? '' : nMod3 === 1 ? '=' : '=='); } function UTF8ArrToStr (aBytes) { var sView = ""; for (var nPart, nLen = aBytes.length, nIdx = 0; nIdx < nLen; nIdx++) { nPart = aBytes[nIdx]; sView += String.fromCharCode( nPart > 251 && nPart < 254 && nIdx + 5 < nLen ? (nPart - 252) * 1073741824 + (aBytes[++nIdx] - 128 >> 18); aBytes[nIdx++] = 128 + (nChr >>> 12 & 63); aBytes[nIdx++] = 128 + (nChr >>> 6 & 63); aBytes[nIdx++] = 128 + (nChr & 63); } else if (nChr < 0x4000000) { aBytes[nIdx++] = 248 + (nChr >>> 24); aBytes[nIdx++] = 128 + (nChr >>> 18 & 63); aBytes[nIdx++] = 128 + (nChr >>> 12 & 63); aBytes[nIdx++] = 128 + (nChr >>> 6 & 63); aBytes[nIdx++] = 128 + (nChr & 63); } else { aBytes[nIdx++] = 252 + (nChr >>> 30); aBytes[nIdx++] = 128 + (nChr >>> 24 & 63); aBytes[nIdx++] = 128 + (nChr >>> 18 & 63); aBytes[nIdx++] = 128 + (nChr >>> 12 & 63); aBytes[nIdx++] = 128 + (nChr >>> 6 & 63); aBytes[nIdx++] = 128 + (nChr & 63); } } return aBytes; } var sMyInput = "Base 64 \u2014 Mozilla Developer Network"; var aMyUTF8Input = strToUTF8Arr(sMyInput); var sMyBase64 = base64EncArr(aMyUTF8Input); alert(sMyBase64); var aMyUTF8Output = base64DecToArr(sMyBase64); var sMyOutput = UTF8ArrToStr(aMyUTF8Output); alert(sMyOutput);These function let us to create also uint8Arrays or arrayBuffers from Base64-encoded strings: var myArray = base64DecToArr("QmFzZSA2NCDigJQgTW96aWxsYSBEZXZlbG9wZXIgTmV0d29yaw=="); var myBuffer = base64DecToArr("QmFzZSA2NCDigJQgTW96aWxsYSBEZXZlbG9wZXIgTmV0d29yaw==").buffer; alert(myBuffer.byteLength); Note: The function base64DecToArr(sBase64[, nBlocksSize]) returns an uint8Array of bytes. If your aim is to build a buffer of 16-bit / 32-bit / 64-bit raw data, use the nBlocksSize argument, which is the number of bytes of which the uint8Array.buffer.bytesLength property must result a multiple (1 or omitted for ASCII, binary strings or UTF-8-encoded strings, 2 for UTF-16 strings, 4 for UTF-32 strings). Group of binary-totext encoding schemes using 64 symbols (plus padding) This article relies too much on references to primary sources. Please improve this by adding secondary or tertiary sources. (April 2019) (Learn how and when to remove this template message) In programming, Base64 is a group of binary-to-text encoding schemes that represent binary data (more specifically, a sequence of 8-bit bytes) in an ASCII string format by translating the data into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding. Each non-final Base64 digit represents exactly 6 bits of data. Three 8-bit bytes (i.e., a total of 24 bits) can therefore be represented by four 6-bit Base64 digits. Common to all binary-to-text encoding schemes, Base64 is designed to carry data stored in binary formats across channels that only reliably support text content. Base64 is particularly prevalent on the World Wide Web[1] where its uses include the ability to embed image files or other binary assets inside textual assets such as HTML and CSS files.[2] Base64 is also widely used for sending e-mail attachments. This is required because SMTP - in its original form - was designed to transport 7-bit ASCII characters only. This encoding causes an overhead of 33?36% (33% by the encoding itself; up to 3% more by the inserted line breaks). Design The particular set of 64 characters chosen to represent the 64-digit values for the base varies between implementations. The general strategy is to choose 64 characters that are common to most encodings and that are also printable. This combination leaves the data unlikely to be modified in transit through information systems, such as email, that were traditionally not 8-bit clean.[3] For example, MIME's Base64 implementation uses A?Z, a?z, and 0?9 for the first 62 values. Other variations share this property but differ in the symbols chosen for the last two values; an example is UTF-7. The earliest instances of this type of encoding were created for dial-up communication between systems running the same OS -- e.g., uuencode for UNIX, BinHex for the TRS-80 (later adapted for the Macintosh) -- and could therefore make more assumptions about what characters were safe to use. For instance, uuencode uses uppercase letters, digits, and many punctuation characters, but no lowercase.[4][5][6][3] Base64 table The Base64 index table: Index Binary Char Index Binary Char Index Binary Char Index Binary Char 0 000000 A 16 010000 Q 32 100000 g 48 110000 w 1 000001 B 17 010001 R 33 100001 h 49 110001 x 2 000010 C 18 010010 S 34 100010 i 50 110010 y 3 000011 D 19 010011 T 35 100011 j 51 110011 z 4 000100 E 20 010100 U 36 100100 k 52 110100 0 5 000101 F 21 010101 V 37 100101 l 53 110101 1 6 000110 G 22 010110 W 38 100110 m 54 110110 2 7 000111 H 23 010111 X 39 100111 n 55 110111 3 8 001000 I 24 011000 Y 40 101000 o 56 111000 4 9 001001 J 25 011001 Z 41 101001 p 57 111001 5 10 001010 K 26 011010 a 42 101010 q 58 111010 6 11 001011 L 27 011011 b 43 101011 r 59 111011 7 12 001100 M 28 011100 c 44 101100 s 60 111100 8 13 001101 N 29 011101 d 45 101101 t 61 111101 9 14 001110 O 30 011110 e 46 101110 u 62 111110 + 15 001111 P 31 011111 f 47 101111 v 63 111111 / Padding = Examples The example below uses ASCII text for simplicity, but this is not a typical use case, as it can already be safely transferred across all systems that can handle Base64. The more typical use is to encode binary data (such as an image); the resulting Base64 data will only contain 64 different ASCII characters, all of which can reliably be transferred across systems that may corrupt the raw source bytes. Here is a quote from Thomas Hobbes's Leviathan: Man is distinguished, not only by his reason, but by this singular passion from other animals, which is a lust of the mind, that by a perseverance of delight in the continued and indefatigable generation of knowledge, exceeds the short vehemence of any carnal pleasure. (Please note that all of the example encodings below use only the bytes shown above; it is not a null-terminated string and has no newline characters.) When that quote is encoded into Base64, it is represented as a byte sequence of 8-bit-padded ASCII characters encoded in MIME's Base64 scheme as follows (newlines and white spaces may be present anywhere but are to be ignored on decoding): TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGlu dWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4= In the above quote, the encoded value of Man is TWFu. Encoded in ASCII, the characters M, a, and n are stored as the byte values 77, 97, and 110, which are the 8-bit binary values 01001101, 01100001, and 01101110. These three values are joined together into a 24-bit string, producing 010011010110000101101110. Groups of 6 bits (6 bits have a maximum of 26 = 64 different binary values) are converted into individual numbers from left to right (in this case, there are four numbers in a 24-bit string), which are then converted into their corresponding Base64 character values. As this example illustrates, Base64 encoding converts three octets into four encoded characters. Source Text (ASCII) M a n Octets 77 (0x4d) 97 (0x61) 110 (0x6e) Bits 0 1 0 0 1 1 0 1 0 1 1 0 0 0 0 1 0 1 1 0 1 1 1 0 Base64encoded Sextets 19 22 5 46 Character T W F u Octets 84 (0x54) 87 (0x57) 70 (0x46) 117 (0x75) = padding characters might be added to make the last encoded block contain four Base64 characters. Hexadecimal to octal transformation is useful to convert between binary and Base64. Both for advanced calculators and programming languages, such conversion is available. For example, the 24 bits above - when converted to hexadecimal - is 4D616E. Those 24 bits - when converted to octal - is 23260556. Those 8 octal digits - when divided into four groups - is 23 26 05 56. Each 2-digit group - when converted to decimal - is 19 22 05 46. Using those four decimal numbers as indices for the Base64 index table, the corresponding ASCII characters are TWFu. If there are only two significant input octets (e.g., 'Ma'), or when the last input group contains only two octets, all 16 bits will be captured in the first three Base64 digits (18 bits); the two least significant bits of the last content-bearing 6-bit block will turn out to be zero, and discarded on decoding (along with the succeeding = padding character): Source Text (ASCII) M a Octets 77 (0x4d) 97 (0x61) Bits 0 1 0 0 1 1 0 1 0 1 1 0 0 0 0 1 0 0 Base64encoded Sextets 19 22 4 Padding Character T W E = Octets 84 (0x54) 87 (0x57) 69 (0x45) 61 (0x3D) If there is only one significant input octet (e.g., 'M'), or when the last input group contains only one octet, all 8 bits will be captured in the first two Base64 digits (12 bits); the four least significant bits of the last content-bearing 6-bit block will turn out to be zero, and discarded on decoding (along with the succeeding two = padding characters): Source Text (ASCII) M Octets 77 (0x4d) Bits 0 1 0 0 1 1 0 1 0 0 0 0 Base64encoded Sextets 19 16 Padding Padding Character T Q = = Octets 84 (0x54) 81 (0x51) 61 (0x3D) 61 (0x3D) Output padding Because Base64 is a six-bit encoding, and because the decoded values are divided into 8-bit octets on a modern computer, every four characters of Base64-encoded text (4 sextets = 4?6 = 24 bits) represents three octets of unencoded text or data (3 octets = 3?8 = 24 bits). This means that when the length of the unencoded input is not a multiple of three, the encoded output must have padding added so that its length is a multiple of four. The padding character is =, which indicates that no further bits are needed to fully encode the input. (This is different from A, which means that the remaining bits are all zeros.) The example below illustrates how truncating the input of the above quote changes the output padding: Input Output Padding Length Text Length Text 20 any carnal pleasure. 28 YW55IGNhcm5hbCBwbGVhc3VyZS4= 1 19 any carnal pleasure 28 YW55IGNhcm5hbCBwbGVhc3VyZQ== 2 18 any carnal pleasur 24 YW55IGNhcm5hbCBwbGVhc3Vy 0 17 any carnal pleasu 24 YW55IGNhcm5hbCBwbGVhc3U= 1 16 any carnal pleas 24 YW55IGNhcm5hbCBwbGVhcw== 2 The padding character is not essential for decoding, since the number of missing bytes can be inferred from the length of the encoded text. In some implementations, the padding character is mandatory, while for others it is not used. An exception in which padding characters are required is when multiple Base64 encoded files have been concatenated. Another consequence of the sextet encoding of octets is that the same octet will be encoded differently depending on its position within a three-octet group of the input, and depending on which particular octet precedes it within the group. For example: Input Output pleasure. cGxlYXN1cmUu leasure. bGVhc3VyZS4= easure. ZWFzdXJlLg== asure. YXN1cmUu sure. c3VyZS4= As the eight bits of an octet are spread across multiple sextets within the output, this is an obvious consequence, since no octet can be stuffed into a single sextet; instead, they must share. However, since the sextets or characters of the output must be saved and manipulated on the same computer system, which only understands octets, they must be represented as octets, with the upper two bits set to zero. (In other words, the encoded output YW55 occupies 4?8 = 32 bits, even though only 24 bits are meaningfully derived from the input, 'any'.) Indeed, these supposedly wasted bits are exactly the reason for the Base64 encoding. The ratio of output bytes to input bytes is 43 (33% overhead). Specifically, given an input of n bytes, the output will be 4 n 3 {\textstyle 4\left\lceil {\frac {n}{3}}\right\rceil } bytes long, including padding characters. Decoding Base64 with padding When decoding Base64 text, four characters are typically converted back to three bytes. The only exceptions are when padding characters exist. A single = indicates that the four characters will decode to only two bytes, while == indicates that the four characters will decode to only a single byte. For example: Encoded Padding Length Decoded YW55IGNhcm5hbCBwbGVhcw== == 1 any carnal pleas YW55IGNhcm5hbCBwbGVhc3U= = 2 any carnal pleasu YW55IGNhcm5hbCBwbGVhc3Vy None 3 any carnal pleasur YW55IGNhcm5hbCBwbGVhc3VyZQ== == 1 any carnal pleasure Decoding Base64 without padding Without padding, after normal decoding of four characters to three bytes over and over again, fewer than four encoded characters may remain. In this situation, only two or three characters can remain. A single remaining encoded character is not possible (because a single Base64 character only contains 6 bits, and 8 bits are required to create a byte, so a minimum of two Base64 characters are required: The first character contributes 6 bits, and the second character contributes its first 2 bits.) For example: Length Encoded Length Decoded 2 YW55IGNhcm5hbCBwbGVhcw 1 any carnal pleas 3 YW55IGNhcm5hbCBwbGVhc3U 2 any carnal pleasu 4 YW55IGNhcm5hbCBwbGVhc3Vy 3 any carnal pleasur Implementations and history Variants summary table Implementations may have some constraints on the alphabet used for representing some bit patterns. This notably concerns the last two characters used in the index table for index 62 and 63, and the character used for padding (which may be mandatory in some protocols or removed in others). The table below summarizes these known variants and provides links to the subsections below. Encoding Encoding characters Separate encoding of lines Decoding non-encoding characters 62nd 63rd pad Separators Length Checksum RFC 1421: Base64 for Privacy-Enhanced Mail (deprecated) + / = mandatory CR+LF 64, or lower for the last line No No RFC 2045: Base64 transfer encoding for MIME + / = mandatory CR+LF At most 76 No Discarded RFC 2152: Base64 for UTF-7 + / No No No RFC 3501: Base64 encoding for IMAP mailbox names + , No No No RFC 4648 ?4: base64 (standard)[a] + / = optional No No RFC 4648 ?5: base64url (URL- and filename-safe standard)[a] - _ = optional No No RFC 4880: Radix-64 for OpenPGP + / = mandatory CR+LF At most 76 Radix-64 encoded 24-bit CRC No ^ a b It is important to note that this variant is intended to provide common features where they are not desired to be specialised by implementations, ensuring robust engineering. This is particularly in light of separate line encodings and restrictions, which have not been considered when previous standards have been co-opted for use elsewhere. Thus, the features indicated here may be over-ridden. Privacy-enhanced mail The first known standardized use of the encoding now called MIME Base64 was in the Privacy-enhanced Electronic Mail (PEM) protocol, proposed by RFC 989 in 1987. PEM defines a "printable encoding" scheme that uses Base64 encoding to transform an arbitrary sequence of octets to a format that can be expressed in short lines of 6-bit characters, as required by transfer protocols such as SMTP.[7] The current version of PEM (specified in RFC 1421) uses a 64-character alphabet consisting of upper- and lower-case Roman letters (A?Z, a?z), the numerals (0?9), and the + and / symbols. The = symbol is also used as a padding suffix.[4] The original specification, RFC 989, additionally used the * symbol to delimit encoded but unencrypted data within the output stream. To convert data to PEM printable encoding, the first byte is placed in the most significant eight bits of a 24-bit buffer, the next in the middle eight, and the third in the least significant eight bits. If there are fewer than three bytes left to encode (or in total), the remaining buffer bits will be zero. The buffer is then used, six bits at a time, most significant first, as indices into the string: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/", and the indicated character is output. The process is repeated on the remaining data until fewer than four octets remain. If three octets remain, they are processed normally. If fewer than three octets (24 bits) are remaining to encode, the input data is right-padded with zero bits to form an integral multiple of six bits. After encoding the non-padded data, if two octets of the 24-bit buffer are padded-zeros, two = characters are appended to the output; if one octet of the 24-bit buffer is filled with padded-zeros, one = character is appended. This signals the decoder that the zero bits added due to padding should be excluded from the reconstructed data. This also guarantees that the encoded output length is a multiple of 4 bytes. PEM requires that all encoded lines consist of exactly 64 printable characters, with the exception of the last line, which may contain fewer printable characters. Lines are delimited by whitespace characters according to local (platformspecific) conventions. MIME Main article: MIME The MIME (Multipurpose Internet Mail Extensions) specification lists Base64 as one of two binary-to-text encoding schemes (the other being quoted-printable).[5] MIME's Base64 encoding is based on that of the RFC 1421 version of PEM: it uses the same 64-character alphabet and encoding mechanism as PEM, and uses the = symbol for output padding in the same way, as described at RFC 2045. MIME does not specify a fixed length for Base64-encoded lines, but it does specify a maximum line length of 76 characters. Additionally it specifies that any extra-alphabetic characters must be ignored by a compliant decoder, although most implementations use a CR/LF newline pair to delimit encoded lines. Thus, the actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original data length, though for very short messages the overhead can be much higher due to the overhead of the headers. Very roughly, the final size of Base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers). The size of the decoded data can be approximated with this formula: bytes = (string_length(encoded_string) - 814) / 1.37 UTF-7 Main article: UTF-7 UTF-7, described first in RFC 1642, which was later superseded by RFC 2152, introduced a system called modified Base64. This data encoding scheme is used to encode UTF-16 as ASCII characters for use in 7-bit transports such as SMTP. It is a variant of the Base64 encoding used in MIME.[8][9] The "Modified Base64" alphabet consists of the MIME Base64 alphabet, but does not use the "=" padding character. UTF-7 is intended for use in mail headers (defined in RFC 2047), and the "=" character is reserved in that context as the escape character for "quoted-printable" encoding. Modified Base64 simply omits the padding and ends immediately after the last Base64 digit containing useful bits leaving up to three unused bits in the last Base64 digit. OpenPGP Main article: OpenPGP OpenPGP, described in RFC 4880, describes Radix-64 encoding, also known as "ASCII armor". Radix-64 is identical to the "Base64" encoding described from MIME, with the addition of an optional 24-bit CRC. The checksum is calculated on the input data before encoding; the checksum is then encoded with the same Base64 algorithm and, prefixed by "=" symbol as separator, appended to the encoded output data.[10] RFC 3548 RFC 3548, entitled The Base16, Base32, and Base64 Data Encodings, is an informational (non-normative) memo that attempts to unify the RFC 1421 and RFC 2045 specifications of Base64 encodings, alternative-alphabet encodings, and the Base32 (which is seldom used) and Base16 encodings. Unless implementations are written to a specification that refers to RFC 3548 and specifically requires otherwise, RFC 3548 forbids implementations from generating messages containing characters outside the encoding alphabet or without padding, and it also declares that decoder implementations must reject data that contain characters outside the encoding alphabet.[6] RFC 4648 This RFC obsoletes RFC 3548 and focuses on Base64/32/16: This document describes the commonly used Base64, Base32, and Base16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. The URL applications Base64 encoding can be helpful when fairly lengthy identifying information is used in an HTTP environment. For example, a database persistence framework for Java objects might use Base64 encoding to encode a relatively large unique id (generally 128-bit UUIDs) into a string for use as an HTTP parameter in HTTP forms or HTTP GET URLs. Also, many applications need to encode binary data in a way that is convenient for inclusion in URLs, including in hidden web form fields, and Base64 is a convenient encoding to render them in a compact way. Using standard Base64 in URL requires encoding of '+', '/' and '=' characters into special percent-encoded hexadecimal sequences ('+' becomes '%2B', '/' becomes '%2F' and '=' becomes '%3D'), which makes the string unnecessarily longer. For this reason, modified Base64 for URL variants exist (such as base64url in RFC 4648), where the '+' and '/' characters of standard Base64 are respectively replaced by '-' and '_', so that using URL encoders/decoders is no longer necessary and has no impact on the length of the encoded value, leaving the same encoded form intact for use in relational databases, web forms, and object identifiers in general. Some variants allow or require omitting the padding '=' signs to avoid them being confused with field separators, or require that any such padding be percent-encoded. Some libraries[which?] will encode '=' to '.', potentially exposing applications to relative path attacks when a folder name is encoded from user data. HTML The atob() and btoa() JavaScript methods, defined in the HTML5 draft specification,[11] provide Base64 encoding and decoding functionality to web pages. The btoa() method outputs padding characters, but these are optional in the input of the atob() method. Other applications Example of an SVG containing embedded JPEG images encoded in Base64[12] Base64 can be used in a variety of contexts: Base64 can be used to transmit and store text that might otherwise cause delimiter collision Spammers use Base64 to evade basic anti-spamming tools, which often do not decode Base64 and therefore cannot detect keywords in encoded messages. Base64 is used to encode character strings in LDIF files Base64 is often used to embed binary data in an XML file, using a syntax similar to ... e.g. favicons in Firefox's exported bookmarks.html. Base64 is used to encode binary files such as images within scripts, to avoid depending on external files. The data URI scheme can use Base64 to represent file contents. For instance, background images and fonts can be specified in a CSS stylesheet file as data: URIs, instead of being supplied in separate files. The FreeSWAN IPSec implementation precedes Base64 strings with 0s, so they can be distinguished from text or hexadecimal strings.[citation needed] Although not part of the official specification for SVG, some viewers can interpret Base64 when used for embedded elements, such as images inside SVG.[13] Radix-64 applications not compatible with Base64 Uuencoding, traditionally used on UNIX, uses ASCII 32 (" " (space)) through 95 ("_"), consecutively, making its 64-character set " !"#$%&'()*+,-./0123456789:;?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_". Avoiding all lower-case letters was helpful because many older printers only printed uppercase. Using consecutive ASCII characters saved computing power because it was only necessary to add 32, not do a lookup. Its use of most punctuation characters and the space character limits its usefulness.[citation needed] BinHex 4 (HQX), which was used within the classic Mac OS, uses a different set of 64 characters. It uses upper and lower case letters, digits, and punctuation characters, but does not use some visually confusable characters like '7', 'O', 'g' and 'o'. Its 64-character set is "!"#$%&'()*+,-012345689@ABCDEFGHIJKLMNPQRSTUVXYZ[`abcdefhijklmpqr". Several other applications use radix-64 sets more similar to but in a different order to the Base64 format, starting with two symbols, then numerals, then uppercase, then lowercase: Unix stores password hashes computed with crypt in the /etc/passwd file using radix-64 encoding called B64. It uses a mostly-alphanumeric set of characters, plus . and /. Its 64-character set is "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz". Padding is not used. The GEDCOM 5.5 standard for genealogical data interchange encodes multimedia files in its text-line hierarchical file format using radix-64. Its 64-character set is also "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".[14] bcrypt hashes are designed to be used in the same way as traditional crypt(3) hashes, and the algorithm uses a similar but permuted alphabet. Its 64-character set is "./ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".[15] Xxencoding uses a mostly-alphanumeric character set similar to crypt and GEDCOM, but using + and - rather than . and /. Its 64-character set is "+-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz". 6PACK, used with some terminal node controllers, uses a different set of 64 characters from 0x00 to 0x3f.[16] Bash supports numeric literals in base 2-64, stretching to a character set of 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ@_.[17] See also 8BITMIME Ascii85 (also called Base85) Base16 Base32 Base36 Base62 Binary-to-text encoding for a comparison of various encoding algorithms Binary number URL References ^ "Base64 encoding and decoding - Web APIs | MDN". ^ "When to base64 encode images (and when not to)". ^ a b The Base16,Base32,and Base64 Data Encodings. IETF. October 2006. doi:10.17487/RFC4648. RFC 4648. Retrieved March 18, 2010. ^ a b Privacy Enhancement for InternetElectronic Mail: Part I: Message Encryption and Authentication Procedures. IETF. February 1993. doi:10.17487/RFC1421. RFC 1421. Retrieved March 18, 2010. ^ a b Multipurpose Internet Mail Extensions: (MIME) Part One: Format of Internet Message Bodies. IETF. November 1996. doi:10.17487/RFC2045. RFC 2045. Retrieved March 18, 2010. ^ a b The Base16, Base32, and Base64 Data Encodings. IETF. July 2003. doi:10.17487/RFC3548. RFC 3548. Retrieved March 18, 2010. ^ Privacy Enhancement for Internet Electronic Mail. IETF. February 1987. doi:10.17487/RFC0989. RFC 989. Retrieved March 18, 2010. ^ UTF-7 A Mail-Safe Transformation Format of Unicode. IETF. July 1994. doi:10.17487/RFC1642. RFC 1642. Retrieved March 18, 2010. ^ UTF-7 A Mail-Safe Transformation Format of Unicode. IETF. May 1997. doi:10.17487/RFC2152. RFC 2152. Retrieved March 18, 2010. ^ OpenPGP Message Format. IETF. November 2007. doi:10.17487/RFC4880. RFC 4880. Retrieved March 18, 2010. ^ "7.3. Base64 utility methods". HTML 5.2 Editor's Draft. World Wide Web Consortium. Retrieved 2 January 2018. Introduced by changeset 5814, 2021-02-01. ^ ^ "Edit fiddle". . ^ "The GEDCOM Standard Release 5.5". Homepages.rootsweb.. Retrieved 2012-06-21. ^ Provos, Niels (1997-02-13). "src/lib/libc/crypt/bcrypt.c r1.1". Retrieved 2018-05-18. ^ "6PACK a "real time" PC to TNC protocol". Retrieved 2013-05-19. ^ "Shell Arithmetic". Bash Reference Manual. Retrieved 8 April 2020. Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. Retrieved from " 2Internet protocol used for relaying e-mails "SMTP" redirects here. For the email delivery company, see SMTP (company). For Short Message Transfer Protocol, see GSM 03.40. Internet protocol suite Application layer BGP DHCP(v6) DNS FTP HTTP HTTPS IMAP LDAP MGCP MQTT NNTP NTP POP PTP ONC/RPC RTP RTSP RIP SIP SMTP SNMP SSH Telnet TLS/SSL XMPP more... Transport layer TCP UDP DCCP SCTP RSVP more... Internet layer IP IPv4 IPv6 ICMP(v6) ECN IGMP IPsec more... Link layer ARP NDP OSPF Tunnels L2TP PPP MAC Ethernet Wi-Fi DSL ISDN FDDI more... vte The Simple Mail Transfer Protocol (SMTP) is an internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP and POP3 are standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync. Since SMTP's introduction in 1981, it has been updated, modified and extended multiple times. The protocol version in common use today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized email addresses. SMTP servers commonly use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications). History Predecessors to SMTP Various forms of one-to-one electronic messaging were used in the 1960s. Users communicated using systems developed for specific mainframe computers. As more computers were interconnected, especially in the U.S. Government's ARPANET, standards were developed to permit exchange of messages between different operating systems. SMTP grew out of these standards developed during the 1970s. SMTP traces its roots to two implementations described in 1971: the Mail Box Protocol, whose implementation has been disputed,[1] but is discussed in RFC 196 and other RFCs, and the SNDMSG program, which, according to RFC 2235, Ray Tomlinson of BBN invented for TENEX computers to send mail messages across the ARPANET.[2][3][4] Fewer than 50 hosts were connected to the ARPANET at this time.[5] Further implementations include FTP Mail[6] and Mail Protocol, both from 1973.[7] Development work continued throughout the 1970s, until the ARPANET transitioned into the modern Internet around 1980. Original SMTP In 1980, Jon Postel published RFC 772 which proposed the Mail Transfer Protocol as a replacement of the use of the File Transfer Protocol (FTP) for mail. RFC 780 of May 1981 removed all references to FTP and allocated port 57 for TCP and UDP[citation needed], an allocation that has since been removed by IANA. In November 1981, Postel published RFC 788 "Simple Mail Transfer Protocol". The SMTP standard was developed around the same time as Usenet, a one-to-many communication network with some similarities.[citation needed] SMTP became widely used in the early 1980s. At the time, it was a complement to the Unix to Unix Copy Program (UUCP), which was better suited for handling email transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time. Both used a store and forward mechanism and are examples of push technology. Though Usenet's newsgroups were still propagated with UUCP between servers,[8] UUCP as a mail transport has virtually disappeared[9] along with the "bang paths" it used as message routing headers.[10] Sendmail, released with 4.1cBSD in 1982, soon after RFC 788 was published in November 1981, was one of the first mail transfer agents to implement SMTP.[11] Over time, as BSD Unix became the most popular operating system on the Internet, Sendmail became the most common MTA (mail transfer agent).[12] The original SMTP protocol supported only unauthenticated unencrypted 7-bit ASCII text communications, susceptible to trivial man-in-the-middle attack, spoofing, and spamming, and requiring any binary data to be encoded to readable text before transmission. Due to absence of a proper authentication mechanism, by design every SMTP server was an open mail relay. The Internet Mail Consortium (IMC) reported that 55% of mail servers were open relays in 1998,[13] but less than 1% in 2002.[14] Because of spam concerns most email providers blocklist open relays,[15] making original SMTP essentially impractical for general use on the Internet. Modern SMTP In November 1995, RFC 1869 defined Extended Simple Mail Transfer Protocol (ESMTP), which established a general structure for all existing and future extensions which aimed to add-in the features missing from the original SMTP. ESMTP defines consistent and manageable means by which ESMTP clients and servers can be identified and servers can indicate supported extensions. Message submission (RFC 2476) and SMTP-AUTH (RFC 2554) were introduced in 1998 and 1999, both describing new trends in email delivery. Originally, SMTP servers were typically internal to an organization, receiving mail for the organization from the outside, and relaying messages from the organization to the outside. But as time went on, SMTP servers (mail transfer agents), in practice, were expanding their roles to become message submission agents for Mail user agents, some of which were now relaying mail from the outside of an organization. (e.g. a company executive wishes to send email while on a trip using the corporate SMTP server.) This issue, a consequence of the rapid expansion and popularity of the World Wide Web, meant that SMTP had to include specific rules and methods for relaying mail and authenticating users to prevent abuses such as relaying of unsolicited email (spam). Work on message submission (RFC 2476) was originally started because popular mail servers would often rewrite mail in an attempt to fix problems in it, for example, adding a domain name to an unqualified address. This behavior is helpful when the message being fixed is an initial submission, but dangerous and harmful when the message originated elsewhere and is being relayed. Cleanly separating mail into submission and relay was seen as a way to permit and encourage rewriting submissions while prohibiting rewriting relay. As spam became more prevalent, it was also seen as a way to provide authorization for mail being sent out from an organization, as well as traceability. This separation of relay and submission quickly became a foundation for modern email security practices. As this protocol started out purely ASCII text-based, it did not deal well with binary files, or characters in many non-English languages. Standards such as Multipurpose Internet Mail Extensions (MIME) were developed to encode binary files for transfer through SMTP. Mail transfer agents (MTAs) developed after Sendmail also tended to be implemented 8-bit-clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary text data (in any 8-bit ASCII-like character encoding) via SMTP. Mojibake was still a problem due to differing character set mappings between vendors, although the email addresses themselves still allowed only ASCII. 8-bit-clean MTAs today tend to support the 8BITMIME extension, permitting some binary files to be transmitted almost as easily as plain text (limits on line length and permitted octet values still apply, so that MIME encoding is needed for most non-text data and some text formats). In 2012, the SMTPUTF8 extension was created to support UTF-8 text, allowing international content and addresses in non-Latin scripts like Cyrillic or Chinese. Many people contributed to the core SMTP specifications, among them Jon Postel, Eric Allman, Dave Crocker, Ned Freed, Randall Gellens, John Klensin, and Keith Moore. Mail processing model Blue arrows depict implementation of SMTP variations Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (mail transfer agent, MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host. Each process is an MTA (an SMTP server) in its own right. The boundary MTA uses DNS to look up the MX (mail exchanger) record for the recipient's domain (the part of the email address on the right of @). The MX record contains the name of the target MTA. Based on the target host and other factors, the sending MTA selects a recipient server and connects to it to complete the mail exchange. Message transfer can occur in a single connection between two MTAs, or in a series of hops through intermediary systems. A receiving SMTP server may be the ultimate destination, an intermediate "relay" (that is, it stores and forwards the message) or a "gateway" (that is, it may forward the message using some protocol other than SMTP). Per RFC 5321 section 2.1, each hop is a formal handoff of responsibility for the message, whereby the receiving server must either deliver the message or properly report the failure to do so. Once the final hop accepts the incoming message, it hands it to a mail delivery agent (MDA) for local delivery. An MDA saves messages in the relevant mailbox format. As with sending, this reception can be done using one or multiple computers, but in the diagram above the MDA is depicted as one box near the mail exchanger box. An MDA may deliver messages directly to storage, or forward them over a network using SMTP or other protocol such as Local Mail Transfer Protocol (LMTP), a derivative of SMTP designed for this purpose. Once delivered to the local mail server, the mail is stored for batch retrieval by authenticated mail clients (MUAs). Mail is retrieved by end-user applications, called email clients, using Internet Message Access Protocol (IMAP), a protocol that both facilitates access to mail and manages stored mail, or the Post Office Protocol (POP) which typically uses the traditional mbox mail file format or a proprietary system such as Microsoft Exchange/Outlook or Lotus Notes/Domino. Webmail clients may use either method, but the retrieval protocol is often not a formal standard. SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header (except trace information) nor the body of the message itself. STD 10 and RFC 5321 define SMTP (the envelope), while STD 11 and RFC 5322 define the message (header and body), formally referred to as the Internet Message Format. Protocol overview SMTP is a connection-oriented, text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP) connection. An SMTP session consists of commands originated by an SMTP client (the initiating agent, sender, or transmitter) and corresponding responses from the SMTP server (the listening agent, or receiver) so that the session is opened, and session parameters are exchanged. A session may include zero or more SMTP transactions. An SMTP transaction consists of three command/reply sequences: MAIL command, to establish the return address, also called return-path,[16] reverse-path,[17] bounce address, mfrom, or envelope sender. RCPT command, to establish a recipient of the message. This command can be issued multiple times, one for each recipient. These addresses are also part of the envelope. DATA to signal the beginning of the message text; the content of the message, as opposed to its envelope. It consists of a message header and a message body separated by an empty line. DATA is actually a group of commands, and the server replies twice: once to the DATA command itself, to acknowledge that it is ready to receive the text, and the second time after the end-of-data sequence, to either accept or reject the entire message. Besides the intermediate reply for DATA, each server's reply can be either positive (2xx reply codes) or negative. Negative replies can be permanent (5xx codes) or transient (4xx codes). A reject is a permanent failure and the client should send a bounce message to the server it received it from. A drop is a positive response followed by message discard rather than delivery. The initiating host, the SMTP client, can be either an end-user's email client, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying message transmissions that resulted in transient failures. A MUA knows the outgoing mail SMTP server from its configuration. A relay server typically determines which server to connect to by looking up the MX (Mail eXchange) DNS resource record for each recipient's domain name. If no MX record is found, a conformant relaying server (not all are) instead looks up the A record. Relay servers can also be configured to use a smart host. A relay server initiates a TCP connection to the server on the "well-known port" for SMTP: port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA is that connecting to an MSA requires SMTP Authentication. SMTP vs mail retrieval SMTP is a delivery protocol only. In normal use, mail is "pushed" to a destination mail server (or next-hop mail server) as it arrives. Mail is routed based on the destination server, not the individual user(s) to which it is addressed. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for use by individual users retrieving messages and managing mail boxes. To permit an intermittently-connected mail server to pull messages from a remote server on demand, SMTP has a feature to initiate mail queue processing on a remote server (see Remote Message Queue Starting below). POP and IMAP are unsuitable protocols for relaying mail by intermittently-connected machines; they are designed to operate after final delivery, when information critical to the correct operation of mail relay (the "mail envelope") has been removed. Remote Message Queue Starting Remote Message Queue Starting enables a remote host to start processing of the mail queue on a server so it may receive messages destined to it by sending a corresponding command. The original TURN command was deemed insecure and was extended in RFC 1985 with the ETRN command which operates more securely using an authentication method based on Domain Name System information.[18] Outgoing mail SMTP server An email client needs to know the IP address of its initial SMTP server and this has to be given as part of its configuration (usually given as a DNS name). This server will deliver outgoing messages on behalf of the user. Outgoing mail server access restrictions Server administrators need to impose some control on which clients can use the server. This enables them to deal with abuse, for example spam. Two solutions have been in common use: In the past, many systems imposed usage restrictions by the location of the client, only permitting usage by clients whose IP address is one that the server administrators control. Usage from any other client IP address is disallowed. Modern SMTP servers typically offer an alternative system that requires authentication of clients by credentials before allowing access. Restricting access by location Under this system, an ISP's SMTP server will not allow access by users who are outside the ISP's network. More precisely, the server may only allow access to users with an IP address provided by the ISP, which is equivalent to requiring that they are connected to the Internet using that same ISP. A mobile user may often be on a network other than that of their normal ISP, and will then find that sending email fails because the configured SMTP server choice is no longer accessible. This system has several variations. For example, an organisation's SMTP server may only provide service to users on the same network, enforcing this by firewalling to block access by users on the wider Internet. Or the server may perform range checks on the client's IP address. These methods were typically used by corporations and institutions such as universities which provided an SMTP server for outbound mail only for use internally within the organisation. However, most of these bodies now use client authentication methods, as described below. Where a user is mobile, and may use different ISPs to connect to the internet, this kind of usage restriction is onerous, and altering the configured outbound email SMTP server address is impractical. It is highly desirable to be able to use email client configuration information that does not need to change. Client authentication Modern SMTP servers typically require authentication of clients by credentials before allowing access, rather than restricting access by location as described earlier. This more flexible system is friendly to mobile users and allows them to have a fixed choice of configured outbound SMTP server. SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the SMTP in order to log in using an authentication mechanism. Ports Communication between mail servers generally uses the standard TCP port 25 designated for SMTP. Mail clients however generally don't use this, instead using specific "submission" ports. Mail services generally accept email submission from clients on one of: 587 (Submission), as formalized in RFC 6409 (previously RFC 2476) 465 This port was deprecated after RFC 2487, until the issue of RFC 8314. Port 2525 and others may be used by some individual providers, but have never been officially supported. Many Internet service providers now block all outgoing port 25 traffic from their customers. Mainly as an anti-spam measure,[19] but also to cure for the higher cost they have when leaving it open, perhaps by charging more from the few customers that require it open. SMTP transport example A typical example of sending a message via SMTP to two mailboxes (alice and theboss) located in the same mail domain ( or ) is reproduced in the following session exchange. (In this example, the conversation parts are prefixed with S: and C:, for server and client, respectively; these labels are not part of the exchange.) After the message sender (SMTP client) establishes a reliable communications channel to the message receiver (SMTP server), the session is opened with a greeting by the server, usually containing its fully qualified domain name (FQDN), in this case smtp.. The client initiates its dialog by responding with a HELO command identifying itself in the command's parameter with its FQDN (or an address literal if none is available).[20] S: 220 smtp. ESMTP Postfix C: HELO relay. S: 250 smtp., I am glad to meet you C: MAIL FROM: S: 250 Ok C: RCPT TO: S: 250 Ok C: RCPT TO: S: 250 Ok C: DATA S: 354 End data with . C: From: "Bob Example" C: To: Alice Example C: Cc: theboss@ C: Date: Tue, 15 Jan 2008 16:02:43 -0500 C: Subject: Test message C: C: Hello Alice. C: This is a test message with 5 header fields and 4 lines in the message body. C: Your friend, C: Bob C: . S: 250 Ok: queued as 12345 C: QUIT S: 221 Bye {The server closes the connection} The client notifies the receiver of the originating email address of the message in a MAIL FROM command. This is also the return or bounce address in case the message cannot be delivered. In this example the email message is sent to two mailboxes on the same SMTP server: one for each recipient listed in the To and Cc header fields. The corresponding SMTP command is RCPT TO. Each successful reception and execution of a command is acknowledged by the server with a result code and response message (e.g., 250 Ok). The transmission of the body of the mail message is initiated with a DATA command after which it is transmitted verbatim line by line and is terminated with an end-of-data sequence. This sequence consists of a new-line (), a single full stop (period), followed by another new-line. Since a message body can contain a line with just a period as part of the text, the client sends two periods every time a line starts with a period; correspondingly, the server replaces every sequence of two periods at the beginning of a line with a single one. Such escaping method is called dot-stuffing. The server's positive reply to the end-of-data, as exemplified, implies that the server has taken the responsibility of delivering the message. A message can be doubled if there is a communication failure at this time, e.g. due to a power shortage: Until the sender has received that 250 reply, it must assume the message was not delivered. On the other hand, after the receiver has decided to accept the message, it must assume the message has been delivered to it. Thus, during this time span, both agents have active copies of the message that they will try to deliver.[21] The probability that a communication failure occurs exactly at this step is directly proportional to the amount of filtering that the server performs on the message body, most often for anti-spam purposes. The limiting timeout is specified to be 10 minutes.[22] The QUIT command ends the session. If the email has other recipients located elsewhere, the client would QUIT and connect to an appropriate SMTP server for subsequent recipients after the current destination(s) had been queued. The information that the client sends in the HELO and MAIL FROM commands are added (not seen in example code) as additional header fields to the message by the receiving server. It adds a Received and Return-Path header field, respectively. Some clients are implemented to close the connection after the message is accepted (250 Ok: queued as 12345), so the last two lines may actually be omitted. This causes an error on the server when trying to send the 221 reply. SMTP Extensions Extension discovery mechanism Clients learn a server's supported options by using the EHLO greeting, as exemplified below, instead of the original HELO. Clients fall back to HELO only if the server does not support EHLO greeting.[23] Modern clients may use the ESMTP extension keyword SIZE to query the server for the maximum message size that will be accepted. Older clients and servers may try to transfer excessively sized messages that will be rejected after consuming network resources, including connect time to network links that is paid by the minute.[24] Users can manually determine in advance the maximum size accepted by ESMTP servers. The client replaces the HELO command with the EHLO command. S: 220 smtp2. ESMTP Postfix C: EHLO bob. S: 250-smtp2. Hello bob. [192.0.2.201] S: 250-SIZE 14680064 S: 250-PIPELINING S: 250 HELP Thus smtp2. declares that it can accept a fixed maximum message size no larger than 14,680,064 octets (8-bit bytes). In the simplest case, an ESMTP server declares a maximum SIZE immediately after receiving an EHLO. According to RFC 1870, however, the numeric parameter to the SIZE extension in the EHLO response is optional. Clients may instead, when issuing a MAIL FROM command, include a numeric estimate of the size of the message they are transferring, so that the server can refuse receipt of overly-large messages. Binary data transfer Original SMTP supports only a single body of ASCII text, therefore any binary data needs to be encoded as text into that body of the message before transfer, and then decoded by the recipient. Binary-to-text encodings, such as uuencode and BinHex were typically used. The 8BITMIME command was developed to address this. It was standardized in 1994 as RFC 1652[25] It facilitates the transparent exchange of e-mail messages containing octets outside the seven-bit ASCII character set by encoding them as MIME content parts, typically encoded with Base64. Mail delivery mechanism extensions On-Demand Mail Relay Main article: On-Demand Mail Relay On-Demand Mail Relay (ODMR) is an SMTP extension standardized in RFC 2645 that allows an intermittently-connected SMTP server to receive email queued for it when it is connected. Internationalization extension Main article: International email Original SMTP supports email addresses composed of ASCII characters only, which is inconvenient for users whose native script is not Latin based, or who use diacritic not in the ASCII character set. This limitation was alleviated via extensions enabling UTF-8 in address names. RFC 5336 introduced experimental[26] UTF8SMTP command and later was superseded by RFC 6531 that introduced SMTPUTF8 command. These extensions provide support for multi-byte and non-ASCII characters in email addresses, such as those with diacritics and other language characters such as Greek and Chinese.[27] Current support is limited, but there is strong interest in broad adoption of RFC 6531 and the related RFCs in countries like China that have a large user base where Latin (ASCII) is a foreign script. Extensions Like SMTP, ESMTP is a protocol used to transport Internet mail. It is used as both an inter-server transport protocol and (with restricted behavior enforced) a mail submission protocol. The main identification feature for ESMTP clients is to open a transmission with the command EHLO (Extended HELLO), rather than HELO (Hello, the original RFC 821 standard). A server will respond with success (code 250), failure (code 550) or error (code 500, 501, 502, 504, or 421), depending on its configuration. An ESMTP server returns the code 250 OK in a multi-line reply with its domain and a list of keywords to indicate supported extensions. A RFC 821 compliant server returns error code 500, allowing ESMTP clients to try either HELO or QUIT. Each service extension is defined in an approved format in subsequent RFCs and registered with the Internet Assigned Numbers Authority (IANA). The first definitions were the RFC 821 optional services: SEND, SOML (Send or Mail), SAML (Send and Mail), EXPN, HELP, and TURN. The format of additional SMTP verbs was set and for new parameters in MAIL and RCPT. Some relatively common keywords (not all of them corresponding to commands) used today are: 8BITMIME ? 8 bit data transmission, RFC 6152 ATRN ? Authenticated TURN for On-Demand Mail Relay, RFC 2645 AUTH ? Authenticated SMTP, RFC 4954 CHUNKING ? Chunking, RFC 3030 DSN ? Delivery status notification, RFC 3461 (See Variable envelope return path) ETRN ? Extended version of remote message queue starting command TURN, RFC 1985 HELP ? Supply helpful information, RFC 821 PIPELINING ? Command pipelining, RFC 2920 SIZE ? Message size declaration, RFC 1870 STARTTLS ? Transport Layer Security, RFC 3207 (2002) SMTPUTF8 ? Allow UTF-8 encoding in mailbox names and header fields, RFC 6531 UTF8SMTP ? Allow UTF-8 encoding in mailbox names and header fields, RFC 5336 (deprecated[28]) The ESMTP format was restated in RFC 2821 (superseding RFC 821) and updated to the latest definition in RFC 5321 in 2008. Support for the EHLO command in servers became mandatory, and HELO designated a required fallback. Non-standard, unregistered, service extensions can be used by bilateral agreement, these services are indicated by an EHLO message keyword starting with "X", and with any additional parameters or verbs similarly marked. SMTP commands are case-insensitive. They are presented here in capitalized form for emphasis only. An SMTP server that requires a specific capitalization method is a violation of the standard.[citation needed] 8BITMIME At least the following servers advertise the 8BITMIME extension: Apache James (since 2.3.0a1)[29] Citadel (since 7.30) Courier Mail Server Gmail[30] IceWarp IIS SMTP Service Kerio Connect Lotus Domino Microsoft Exchange Server (as of Exchange Server 2000) Novell GroupWise OpenSMTPD Oracle Communications Messaging Server Postfix Sendmail (since 6.57) The following servers can be configured to advertise 8BITMIME, but do not perform conversion of 8-bit data to 7-bit when connecting to non-8BITMIME relays: Exim and qmail do not translate eight-bit messages to seven-bit when making an attempt to relay 8-bit data to non-8BITMIME peers, as is required by the RFC.[31] This does not cause problems in practice, since virtually all modern mail relays are 8-bit clean.[32] Microsoft Exchange Server 2003 advertises 8BITMIME by default, but relaying to a non-8BITMIME peer results in a bounce. This is allowed by RFC 6152 section 3. SMTP-AUTH Main article: SMTP Authentication The SMTP-AUTH extension provides an access control mechanism. It consists of an authentication step through which the client effectively logs into the mail server during the process of sending mail. Servers that support SMTP-AUTH can usually be configured to require clients to use this extension, ensuring the true identity of the sender is known. The SMTP-AUTH extension is defined in RFC 4954. SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as spammers. It does not necessarily guarantee the authenticity of either the SMTP envelope sender or the RFC 2822 "From:" header. For example, spoofing, in which one sender masquerades as someone else, is still possible with SMTP-AUTH unless the server is configured to limit message from-addresses to addresses this AUTHed user is authorized for. The SMTP-AUTH extension also allows one mail server to indicate to another that the sender has been authenticated when relaying mail. In general this requires the recipient server to trust the sending server, meaning that this aspect of SMTP-AUTH is rarely used on the Internet.[citation needed] SMTPUTF8 Supporting servers include: Postfix (version 3.0 and later)[33] Momentum (versions 4.1[34] and 3.6.5, and later) Sendmail (under development) Exim (experimental as of the 4.86 release) CommuniGate Pro as of version 6.2.2[35] Courier-MTA as of version 1.0[36] Halon as of version 4.0[37] Microsoft Exchange Server as of protocol revision 14.0[38] Haraka and other servers.[39] Oracle Communications Messaging Server as of release 8.0.2.[40] Security extensions Mail delivery can occur both over plain text and encrypted connections, however the communicating parties might not know in advance of other party's ability to use secure channel. STARTTLS or "Opportunistic TLS" Main articles: Opportunistic TLS and Email encryption The STARTTLS extensions enables supporting SMTP servers to notify connecting clients that it supports TLS encrypted communication and offers the opportunity for clients to upgrade their connection by sending the STARTTLS command. Servers supporting the extension do not inherently gain any security benefits from its implementation on its own, as upgrading to a TLS encrypted session is dependent on the connecting client deciding to exercise this option, hence the term opportunistic TLS. STARTTLS is effective only against passive observation attacks, since the STARTTLS negotiation happens in plain text and an active attacker can trivially remove STARTTLS commands. This type of man-in-the-middle attack is sometimes referred to as STRIPTLS, where the encryption negotiation information sent from one end never reaches the other. In this scenario both parties take the invalid or unexpected responses as indication that the other does not properly support STARTTLS, defaulting to traditional plain-text mail transfer.[41] Note that STARTTLS is also defined for IMAP and POP3 in other RFCs, but these protocols serve different purposes: SMTP is used for communication between message transfer agents, while IMAP and POP3 are for end clients and message transfer agents. Electronic Frontier Foundation maintains a "STARTTLS Everywhere" list that similarly to "HTTPS Everywhere" list allows relying parties to discover others supporting secure communication without prior communication.[42]

RFC 8314 officially declared plain text obsolete and recommend always using TLS, adding ports with implicit TLS. SMTP MTA Strict Transport Security A newer 2018 RFC 8461called "SMTP MTA Strict Transport Security (MTA-STS)" aims to address the problem of active adversary by defining a protocol for mail servers to declare their ability to use secure channels in specific files on the server and specific DNS TXT records. The relying party would regularly check existence of such record, and cache it for the amount of time specified in the record and never communicate over insecure channels until record expires.[41] Note that MTA-STS records apply only to SMTP traffic between mail servers while communications between end client and the mail server are protected by HTTPS, HTTP Strict Transport Security. In April 2019 Google Mail announced support for MTA-STS.[43] SMTP TLS Reporting A number of protocols allows secure delivery of messages, but they can fail due to misconfigurations or deliberate active interference, leading to undelivered messages or delivery over unencrypted or unauthenticated channels. RFC 8460 "SMTP TLS Reporting" describes a reporting mechanism and format for sharing statistics and specific information about potential failures with recipient domains. Recipient domains can then use this information to both detect potential attacks and diagnose unintentional misconfigurations. In April 2019 Google Mail announced support for SMTP TLS Reporting.[43] Spoofing and spamming Main articles: Anti-spam techniques and Email authentication The original design of SMTP had no facility to authenticate senders, or check that servers were authorized to send on their behalf, with the result that email spoofing is possible, and commonly used in email spam and phishing. Occasional proposals are made to modify SMTP extensively or replace it completely. One example of this is Internet Mail 2000, but neither it, nor any other has made much headway in the face of the network effect of the huge installed base of classic SMTP. Instead, mail servers now use a range of techniques, such as stricter enforcement of standards such as RFC 5322,[44][45] DomainKeys Identified Mail, Sender Policy Framework and DMARC, DNSBLs and greylisting to reject or quarantine suspicious emails.[46] Implementations There is also SMTP proxy implementation as for example nginx.[47] Main articles: List of mail server software and Comparison of mail servers Related requests for comments RFC 1123 ? Requirements for Internet Hosts--Application and Support (STD 3) RFC 1870 ? SMTP Service Extension for Message Size Declaration (bsoletes: RFC 1653) RFC 2505 ? Anti-Spam Recommendations for SMTP MTAs (BCP 30) RFC 2821 ? Simple Mail Transfer Protocol RFC 2920 ? SMTP Service Extension for Command Pipelining (STD 60) RFC 3030 ? SMTP Service Extensions for Transmission of Large and Binary MIME Messages RFC 3207 ? SMTP Service Extension for Secure SMTP over Transport Layer Security (obsoletes RFC 2487) RFC 3461 ? SMTP Service Extension for Delivery Status Notifications (obsoletes RFC 1891) RFC 3463 ? Enhanced Status Codes for SMTP (obsoletes RFC 1893, updated by RFC 5248) RFC 3464 ? An Extensible Message Format for Delivery Status Notifications (obsoletes RFC 1894) RFC 3798 ? Message Disposition Notification (updates RFC 3461) RFC 3834 ? Recommendations for Automatic Responses to Electronic Mail RFC 3974 ? SMTP Operational Experience in Mixed IPv4/v6 Environments RFC 4952 ? Overview and Framework for Internationalized Email (updated by RFC 5336) RFC 4954 ? SMTP Service Extension for Authentication (obsoletes RFC 2554, updates RFC 3463, updated by RFC 5248) RFC 5068 ? Email Submission Operations: Access and Accountability Requirements (BCP 134) RFC 5248 ? A Registry for SMTP Enhanced Mail System Status Codes (BCP 138) (updates RFC 3463) RFC 5321 ? The Simple Mail Transfer Protocol (obsoletes RFC 821 aka STD 10, RFC 974, RFC 1869, RFC 2821, updates RFC 1123) RFC 5322 ? Internet Message Format (obsoletes RFC 822 aka STD 11, and RFC 2822) RFC 5504 ? Downgrading Mechanism for Email Address Internationalization RFC 6409 ? Message Submission for Mail (STD 72) (obsoletes RFC 4409, RFC 2476) RFC 6522 ? The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages (obsoletes RFC 3462, and in turn RFC 1892) RFC 6531 ? SMTP Extension for Internationalized Email Addresses (updates RFC 2821, RFC 2822, RFC 4952, and RFC 5336) RFC 8314 ? Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access See also Bounce address CRAM-MD5 (a SASL mechanism for ESMTPA) RFC 2195 Email Email encryption DKIM Ident List of mail server software List of SMTP server return codes POP before SMTP / SMTP after POP Internet Message Access Protocol Binary Content Extension RFC 3516 Sender Policy Framework (SPF) Simple Authentication and Security Layer (SASL) RFC 4422 SMTP Authentication Variable envelope return path Comparison of email clients for information about SMTP support Notes ^ The History of Electronic Mail, Tom Van Vleck: "It is not clear this protocol was ever implemented" ^ The First Network Email, Ray Tomlinson, BBN ^ Picture of "The First Email Computer" by Dan Murphy, a PDP-10 ^ Dan Murphy's TENEX and TOPS-20 Papers Archived November 18, 2007, at the Wayback Machine ^ RFC 2235 ^ RFC 469 ? Network Mail Meeting Summary ^ RFC 524 ? A Proposed Mail Protocol ^ ^ draft-barber-uucp-project-conclusion-05 ? The Conclusion of the UUCP Mapping Project ^ The article about sender rewriting contains technical background info about the early SMTP history and source routing before RFC 1123. ^ Eric Allman (1983), Sendmail ? An Internetwork Mail Router (PDF), BSD UNIX documentation set, Berkeley: University of California, retrieved June 29, 2012 ^ Craig Partridge (2008), The Technical Development of Internet Email (PDF), IEEE Annals of the History of Computing, 30, IEEE Computer Society, pp. 3?29, doi:10.1109/MAHC.2008.32, S2CID 206442868, archived from the original (PDF) on May 12, 2011 ^ Paul Hoffman (February 1, 1998). "Allowing Relaying in SMTP: A Survey". Internet Mail Consortium. Retrieved May 30, 2010. ^ Paul Hoffman (August 2002). "Allowing Relaying in SMTP: A Series of Surveys". Internet Mail Consortium. Archived from the original on January 18, 2007. Retrieved May 30, 2010. ^ "In Unix, what is an open mail relay? - Knowledge Base". web.. June 17, 2007. Retrieved March 15, 2021. ^ "The MAIL, RCPT, and DATA verbs", [D. J. Bernstein] ^ RFC 5321 Section-7.2 ^ Systems, Message. "Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities". . Retrieved July 19, 2020. ^ Cara Garretson (2005). "ISPs Pitch In to Stop Spam". PC World. Retrieved January 18, 2016. Last month, the Anti-Spam Technical Alliance, formed last year by Yahoo, America Online, EarthLink, and Microsoft, issued a list of antispam recommendations that includes filtering Port 25. ^ RFC 5321, Simple Mail Transfer Protocol, J. Klensin, The Internet Society (October 2008) ^ RFC 1047 ^ rfc5321#section-4.5.3.2.6 ^ John Klensin; Ned Freed; Marshall T. Rose; Einar A. Stefferud; Dave Crocker (November 1995). SMTP Service Extensions. IETF. doi:10.17487/RFC1869. RFC 1869. ^ "MAIL Parameters". IANA. Retrieved April 3, 2016. ^ Which was obsoleted in 2011 by RFC 6152 corresponding to the then new STD 71 ^ "MAIL Parameters". November 15, 2018. ^ Jiankang Yao (December 19, 2014). "Chinese email address". EAI (Mailing list). IETF. Retrieved May 24, 2016. ^ "SMTP Service Extension Parameters". IANA. Retrieved November 5, 2013. ^ James Server - ChangeLog. James.. Retrieved on 2013-07-17. ^ 8BITMIME service advertised in response to EHLO on gmail-smtp-in.l. port 25, checked 23 November 2011 ^ Qmail bugs and wishlist. Home.pages.de. Retrieved on 2013-07-17. ^ The 8BITMIME extension. Cr.yp.to. Retrieved on 2013-07-17. ^ "Postfix SMTPUTF8 support is enabled by default", February 8, 2015, ^ "Message Systems Introduces Latest Version Of Momentum With New API-Driven Capabilities" (Press release). ^ "Version 6.2 Revision History". . ^ Sam Varshavchik (September 18, 2018). "New releases of Courier packages". courier-announce (Mailing list). ^ changelog ^ "MS-OXSMTP: Simple Mail Transfer Protocol (SMTP) Extensions". July 24, 2018. ^ "EAI Readiness in TLDs" (PDF). February 12, 2019. ^ "Communications Messaging Server Release Notes". . October 2017. ^ a b "Introducing MTA Strict Transport Security (MTA-STS) | Hardenize Blog". . Retrieved April 25, 2019. ^ "STARTTLS Everywhere". EFF. Retrieved August 15, 2019. ^ a b Cimpanu, Catalin. "Gmail becomes first major email provider to support MTA-STS and TLS Reporting". ZDNet. Retrieved April 25, 2019. ^ Message Non Compliant with RFC 5322 ^ Message could not be delivered. Please ensure the message is RFC 5322 compliant. ^ Why are the emails sent to Microsoft Account rejected for policy reasons? ^ "NGINX Docs | Configuring NGINX as a Mail Proxy Server". References Hughes, L (1998). Internet E-mail: Protocols, Standards and Implementation. Artech House Publishers. ISBN 978-0-89006-939-4. Hunt, C (2003). sendmail Cookbook. O'Reilly Media. ISBN 978-0-596-00471-2. Johnson, K (2000). Internet Email Protocols: A Developer's Guide. Addison-Wesley Professional. ISBN 978-0-201-43288-6. Loshin, P (1999). Essential Email Standards: RFCs and Protocols Made Practical. John Wiley & Sons. ISBN 978-0-471-34597-8. Rhoton, J (1999). Programmer's Guide to Internet Mail: SMTP, POP, IMAP, and LDAP. Elsevier. ISBN 978-1-55558-212-8. Wood, D (1999). Programming Internet Mail. O'Reilly. ISBN 978-1-56592-479-6. External links IANA registry of mail parameters includes service extension keywords RFC 1869 SMTP Service Extensions RFC 5321 Simple Mail Transfer Protocol RFC 4954 SMTP Service Extension for Authentication (obsoletes RFC 2554) RFC 3848 SMTP and LMTP Transmission Types Registration (with ESMTPA) RFC 6409 Message Submission for Mail (obsoletes RFC 4409, which obsoletes RFC 2476) Retrieved from "

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