openssl/doc/crypto/EVP_EncodeInit.pod

=pod

=head1 NAME

EVP_ENCODE_CTX_new, EVP_ENCODE_CTX_free, EVP_ENCODE_CTX_num, EVP_EncodeInit,
EVP_EncodeUpdate, EVP_EncodeFinal, EVP_EncodeBlock, EVP_DecodeInit,
EVP_DecodeUpdate, EVP_DecodeFinal, EVP_DecodeBlock - EVP base 64 encode/decode
routines

=head1 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_num(EVP_ENCODE_CTX *ctx);
 void EVP_EncodeInit(EVP_ENCODE_CTX *ctx);
 void 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);

=head1 DESCRIPTION

The EVP encode routines provide a high level interface to base 64 encoding and
decoding. Base 64 encoding converts binary data into a printable form that uses
the characters A-Z, a-z, 0-9, "+" and "/" to represent the data. For every 3
bytes of binary data provided 4 bytes of base 64 encoded data will be produced
plus some occasional newlines (see below). If the input data length is not a
multiple of 3 then the output data will be padded at the end using the "="
character.

EVP_ENCODE_CTX_new() allocates, initializes and returns a context to be used for
the encode/decode functions.

EVP_ENCODE_CTX_free() cleans up an encode/decode context B<ctx> and frees up the
space allocated to it.

Encoding of binary data is performed in blocks of 48 input bytes (or less for
the final block). For each 48 byte input block encoded 64 bytes of base 64 data
is output plus an additional newline character (i.e. 65 bytes in total). The
final block (which may be less than 48 bytes) will output 4 bytes for every 3
bytes of input. If the data length is not divisible by 3 then a full 4 bytes is
still output for the final 1 or 2 bytes of input. Similarly a newline character
will also be output.

EVP_EncodeInit() initialises B<ctx> for the start of a new encoding operation.

EVP_EncodeUpdate() encode B<inl> bytes of data found in the buffer pointed to by
B<in>. The output is stored in the buffer B<out> and the number of bytes output
is stored in B<*outl>. It is the caller's responsibility to ensure that the
buffer at B<out> is sufficiently large to accommodate the output data. Only full
blocks of data (48 bytes) will be immediately processed and output by this
function. Any remainder is held in the B<ctx> object and will be 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 B<inl> with the
amount of unprocessed data held in B<ctx> and divide the result by 48 (ignore
any remainder). This gives the number of blocks of data that will be processed.
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() will set B<*outl> to 0.

EVP_EncodeFinal() must be called at the end of an encoding operation. It will
process any partial block of data remaining in the B<ctx> object. The output
data will be stored in B<out> and the length of the data written will be stored
in B<*outl>. It is the caller's responsibility to ensure that B<out> is
sufficiently large to accommodate the output data which will never be more than
65 bytes plus an additional NUL terminator (i.e. 66 bytes in total).

EVP_ENCODE_CTX_num() will return the number of as yet unprocessed bytes still to
be encoded or decoded that are pending in the B<ctx> object.

EVP_EncodeBlock() encodes a full block of input data in B<f> and of length
B<dlen> and stores it in B<t>. For every 3 bytes of input provided 4 bytes of
output data will be produced. If B<dlen> is not divisible by 3 then the block is
encoded as a final block of data and the output is padded such that it is always
divisible by 4. Additionally a NUL terminator character will be added. For
example if 16 bytes of input data is provided then 24 bytes of encoded data is
created plus 1 byte for a NUL terminator (i.e. 25 bytes in total). The length of
the data generated I<without> the NUL terminator is returned from the function.

EVP_DecodeInit() initialises B<ctx> for the start of a new decoding operation.

EVP_DecodeUpdate() decodes B<inl> characters of data found in the buffer pointed
to by B<in>. The output is stored in the buffer B<out> and the number of bytes
output is stored in B<*outl>. It is the caller's responsibility to ensure that
the buffer at B<out> is sufficiently large to accommodate the output data. This
function will attempt to decode as much data as possible in 4 byte chunks. Any
whitespace, newline or carriage return characters are ignored. Any partial chunk
of unprocessed data (1, 2 or 3 bytes) that remains at the end will be held in
the B<ctx> object and processed by a subsequent call to EVP_DecodeUpdate(). If
any illegal base 64 characters are encountered or if the base 64 padding
character "=" is encountered in the middle of the data then the function 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
input data characters processed included the base 64 padding character "=" and
therefore no more non-padding character data is expected to be processed. For
every 4 valid base 64 bytes processed (ignoring whitespace, carriage returns and
line feeds), 3 bytes of binary output data will be produced (or less at the end
of the data where the padding character "=" has been used).

EVP_DecodeFinal() must be called at the end of a decoding operation. If there
is any unprocessed data still in B<ctx> then the input data must not have been
a multiple of 4 and therefore an error has occurred. The function will return -1
in this case. Otherwise the function returns 1 on success.

EVP_DecodeBlock() will decode the block of B<n> characters of base 64 data
contained in B<f> and store the result in B<t>. Any leading whitespace will be
trimmed as will any trailing whitespace, newlines, carriage returns or EOF
characters. After such trimming the length of the data in B<f> must be divisbile
by 4. For every 4 input bytes exactly 3 output bytes will be produced. The
output will be padded with 0 bits if necessary to ensure that the output is
always 3 bytes for every 4 input bytes. This function will return the length of
the data decoded or -1 on error.

=head1 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
B<ctx>.

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 base 64 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.

=head1 SEE ALSO

L<evp(3)>

=head1 COPYRIGHT

Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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
L<https://www.openssl.org/source/license.html>.

=cut