EVP_ENCODEINIT(3ossl)

🌸 EVP_ENCODEINIT 🌸

okay bestie, we're doing the OpenSSL 3ossl manual page today — the base64 encode/decode crew. yes it's a lot of function names, no I'm not scared, let's go 💅

✨ NAME ✨

EVP_ENCODE_CTX_new, EVP_ENCODE_CTX_free, EVP_ENCODE_CTX_copy, EVP_ENCODE_CTX_num, EVP_EncodeInit, EVP_EncodeUpdate, EVP_EncodeFinal, EVP_EncodeBlock, EVP_DecodeInit, EVP_DecodeUpdate, EVP_DecodeFinal, EVP_DecodeBlock — the EVP base64 encode/decode routines 💖

🦋 SYNOPSIS 🦋

#include <openssl/evp.h>

EVP_ENCODE_CTX *EVP_ENCODE_CTX_new(void);
void EVP_ENCODE_CTX_free(EVP_ENCODE_CTX *ctx);
int EVP_ENCODE_CTX_copy(EVP_ENCODE_CTX *dctx, EVP_ENCODE_CTX *sctx);
int EVP_ENCODE_CTX_num(EVP_ENCODE_CTX *ctx);
void EVP_EncodeInit(EVP_ENCODE_CTX *ctx);
int EVP_EncodeUpdate(EVP_ENCODE_CTX *ctx, unsigned char *out, int *outl,
                     const unsigned char *in, int inl);
void EVP_EncodeFinal(EVP_ENCODE_CTX *ctx, unsigned char *out, int *outl);
int EVP_EncodeBlock(unsigned char *t, const unsigned char *f, int n);

void EVP_DecodeInit(EVP_ENCODE_CTX *ctx);
int EVP_DecodeUpdate(EVP_ENCODE_CTX *ctx, unsigned char *out, int *outl,
                     const unsigned char *in, int inl);
int EVP_DecodeFinal(EVP_ENCODE_CTX *ctx, unsigned char *out, int *outl);
int EVP_DecodeBlock(unsigned char *t, const unsigned char *f, int n);

🎀 DESCRIPTION 🎀

so the EVP encode routines are basically the high-level, main-character interface to base64 encoding and decoding ✨ Base64 encoding turns your binary data into something printable, using the characters A-Z, a-z, 0-9, "+" and "/" to represent the data. For every 3 bytes of binary data you feed in, 4 bytes of base64 encoded data come out, plus some occasional newlines (more on that below, iykyk). And if your input data length isn't a multiple of 3, the output gets padded at the end with the "=" character. cute, right? 🎀

• EVP_ENCODE_CTX_new() — allocates, initializes and returns a context for you to use with the encode/decode functions.
• EVP_ENCODE_CTX_free() — cleans up an encode/decode context ctx and frees up the space allocated to it. If the argument is NULL, literally nothing happens. no notes. 💅

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okay so the encoding itself? binary data gets encoded in blocks of 48 input bytes (or less for the final block). For every 48-byte input block, 64 bytes of base64 data are output plus an additional newline character — that's 65 bytes in total, if you're counting 🔥 The final block (which may be less than 48 bytes) outputs 4 bytes for every 3 bytes of input. If the data length isn't divisible by 3, then a full 4 bytes is still output for the final 1 or 2 bytes of input. And similarly, a newline character also gets output. consistent queen 🌈

• EVP_EncodeInit() — initialises ctx for the start of a new encoding operation.
• EVP_EncodeUpdate() — encodes inl bytes of data found in the buffer pointed to by in. The output goes into the buffer out, and the number of bytes output is stored in *outl. ⚠️ It is the caller's responsibility to make sure the buffer at out is sufficiently large to accommodate the output data. Only full blocks of data (48 bytes) get immediately processed and output by this function. Any remainder is held in the ctx object and gets processed by a subsequent call to EVP_EncodeUpdate() or EVP_EncodeFinal(). To calculate the required size of the output buffer: add together the value of inl with the amount of unprocessed data held in ctx, then divide the result by 48 (ignore any remainder). That gives you the number of blocks of data that will be processed. Then ensure the output buffer contains 65 bytes of storage for each block, plus an additional byte for a NUL terminator. EVP_EncodeUpdate() may be called repeatedly to process large amounts of input data. In the event of an error, EVP_EncodeUpdate() sets *outl to 0 and returns 0. On success, 1 is returned.
• EVP_EncodeFinal() — must be called at the end of an encoding operation. It processes any partial block of data remaining in the ctx object. The output data is stored in out and the length of the data written is stored in *outl. Again, it's the caller's job to make sure out is sufficiently large — it will never be more than 65 bytes plus an additional NUL terminator (i.e. 66 bytes in total).
• EVP_ENCODE_CTX_copy() — copies a context sctx to a context dctx. 💀 dctx must be initialized before calling this function.
• EVP_ENCODE_CTX_num() — returns the number of as-yet unprocessed bytes still to be encoded or decoded that are pending in the ctx object. like a little to-do list count 🫶
• EVP_EncodeBlock() — encodes a full block of input data in f of length n and stores it in t. For every 3 bytes of input provided, 4 bytes of output data are produced. If n isn't divisible by 3, then the block is encoded as a final block of data and the output is padded so it's always divisible by 4. Additionally, a NUL terminator character gets added. For example: if 16 bytes of input data is provided, then 24 bytes of encoded data are created plus 1 byte for a NUL terminator (i.e. 25 bytes in total). The length of the data generated without the NUL terminator is what the function returns. math ate 💅

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decoding time, bestie, this is where it gets spicy 🌶️

• EVP_DecodeInit() — initialises ctx for the start of a new decoding operation.
• EVP_DecodeUpdate() — decodes inl characters of data found in the buffer pointed to by in. The output is stored in the buffer out and the number of bytes output is stored in *outl. (Caller's responsibility to make sure out is big enough — you know the drill by now 🙈.) This function attempts to decode as much data as possible in chunks of up to 80 base64 characters at a time. Residual input shorter than the internal chunk size gets buffered in ctx if its length isn't a multiple of 4 (including any padding), and will be processed in future calls to EVP_DecodeUpdate() or EVP_DecodeFinal(). If the final chunk length is a multiple of 4, it's decoded immediately and not buffered. iconic behaviour honestly ✨

Any whitespace, newline or carriage return characters are ignored. For compatibility with PEM, the - (hyphen) character is treated as a soft end-of-input — subsequent bytes are not buffered, and the return value will be 0 to indicate that the end of the base64 input has been detected. ⚠️ The soft end-of-input, if present, MUST occur after a multiple of 4 valid base64 input bytes. Also, the soft end-of-input condition is not remembered in ctx — it's up to the caller to avoid further calls to EVP_DecodeUpdate() after a 0 or negative (error) return. no cap, that's on you 🫵

If any invalid base64 characters are encountered, or if the base64 padding character (=) is encountered in the middle of the data, then EVP_DecodeUpdate() returns -1 to indicate an error. A return value of 0 or 1 indicates successful processing of the data. A return value of 0 additionally indicates that the last 4 bytes processed ended with base64 padding (=), or that the next 4-byte group starts with the soft end-of-input (-) character — and therefore no more input data is expected to be processed.

For every 4 valid base64 bytes processed (ignoring whitespace, carriage returns and line feeds), 3 bytes of binary output data will be produced — except at the end of data terminated with one or two padding characters. 🧚

• EVP_DecodeFinal() — should be called at the end of a decoding operation, but it will never decode additional data. If there's no residual data, it returns 1 to indicate success. If there is residual data and its length isn't a multiple of 4 — i.e. it wasn't properly padded — then -1 is returned in that case to indicate an error. (yes, the original says "-1 is is returned", we're keeping the double "is" energy, it's giving typo chic 😭)
• EVP_DecodeBlock() — decodes the block of n characters of base64 data contained in f and stores the result in t. Any leading whitespace gets trimmed, as does any trailing whitespace, newlines, carriage returns or EOF characters. ⚠️ Internal whitespace MUST NOT be present. After trimming, the data in f MUST consist entirely of valid base64 characters or padding (only at the tail of the input) and its length MUST be divisible by 4. For every 4 input bytes, exactly 3 output bytes are produced. Padding bytes (=) — even if internal — are decoded to 6 zero bits, so the caller is responsible for taking trailing padding into account by ignoring as many bytes at the tail of the returned output. EVP_DecodeBlock() returns the length of the data decoded, or -1 on error.

🔥 RETURN VALUES 🔥

• EVP_ENCODE_CTX_new() — returns a pointer to the newly allocated EVP_ENCODE_CTX object, or NULL on error.
• EVP_ENCODE_CTX_num() — returns the number of bytes pending encoding or decoding in ctx.
• EVP_EncodeUpdate() — returns 0 on error or 1 on success.
• EVP_EncodeBlock() — returns the number of bytes encoded, excluding the NUL terminator.
• EVP_DecodeUpdate() — returns -1 on error and 0 or 1 on success. If 0 is returned, then no more non-padding base64 characters are expected.
• EVP_DecodeFinal() — returns -1 on error or 1 on success.
• EVP_DecodeBlock() — returns the length of the data decoded or -1 on error.

🌈 SEE ALSO 🌈

evp(7) — go read it, bestie, it's the whole family 💜

👑 HISTORY 👑

The EVP_DecodeUpdate() function was fixed in OpenSSL 3.5, so now it produces the number of bytes specified in outl* and does not decode padding bytes (=) to 6 zero bits. glow-up confirmed ✨

💖 COPYRIGHT 💖

Copyright 2016-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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and that's the whole page, bestie 🫶 this was EVP_ENCODEINIT(3ossl) from OpenSSL 3.5.6, dated 2026-04-07. go forth and encode responsibly 💅🎀