Add documentation for EVP_EncodeInit() and similar functions

=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)>

=cut

implementation. However, new applications should not typically use this (preferring, for example,

PBKDF2 from PCKS#5).

The L<B<EVP_Encode>I<...>|EVP_EncodeInit(3)> and

L<B<EVP_Decode>I<...>|EVP_EncodeInit(3)> functions implement base 64 encoding

and decoding.

All the symmetric algorithms (ciphers), digests and asymmetric algorithms

(public key algorithms) can be replaced by L<engine(3)> modules providing alternative

implementations. If ENGINE implementations of ciphers or digests are registered

L<EVP_DigestSignInit(3)>,

L<EVP_SignInit(3)>,

L<EVP_VerifyInit(3)>,

L<EVP_EncodeInit(3)>,

L<EVP_PKEY_new(3)>,

L<EVP_PKEY_set1_RSA(3)>,

L<EVP_PKEY_keygen(3)>,