ENCODING SCHEMES

Binary-to-text encoding schemes are specified by ENUMs in the VMOD's constructor, methods and functions. Decodings convert a (possibly concatenated) string into a blob, while encodings convert a blob into a string.

ENUM values for an encoding scheme can be one of:

Empty strings are decoded into a "null blob" (of length 0), and conversely a null blob is encoded as the empty string.

For encodings with HEX or URL, you may also specify a case ENUM with one of the values LOWER, UPPER or DEFAULT to produce a string with lower- or uppercase hex digits (in [a-f] or [A-F]). The default value for case is DEFAULT, which for HEX and URL means the same as LOWER.

The case ENUM is not relevant for decodings; HEX or URL strings to be decoded as BLOBs may have hex digits in either case, or in mixed case.

The case ENUM MUST be set to DEFAULT for the other encodings (BASE64* and IDENTITY). You cannot, for example, produce an uppercase string by using the IDENTITY scheme with case=UPPER. To change the case of a string, use the std.toupper() or std.tolower() functions from vmod_std(3).

IDENTITY

The simplest encoding converts between the BLOB and STRING data types, leaving the contents byte-identical.

Note that a BLOB may contain a null byte at any position before its end; if such a BLOB is decoded with IDENTITY, the resulting STRING will have a null byte at that position. Since VCL strings, like C strings, are represented with a terminating null byte, the string will be truncated, appearing to contain less data than the original blob. For example:

# Decode from the hex encoding for "foo\0bar".
# The header will be seen as "foo".
set resp.http.Trunced-Foo1


    = blob.encode(IDENTITY, blob=blob.decode(HEX,


                                             encoded="666f6f00626172"));

IDENTITY is the default encoding and decoding. So the above can also be written as:

# Decode from the hex encoding for "foo\0bar".
# The header will be seen as "foo".
set resp.http.Trunced-Foo2


  = blob.encode(blob=blob.decode(HEX, encoded="666f6f00626172"));

The case ENUM MUST be set to DEFAULT for IDENTITY encodings.

BASE64*

The base64 encoding schemes use 4 characters to encode 3 bytes. There are no newlines or maximal line lengths -- whitespace is not permitted.

The BASE64 encoding uses the alphanumeric characters, + and /; and encoded strings are padded with the = character so that their length is always a multiple of four.

The BASE64URL encoding also uses the alphanumeric characters, but - and _ instead of + and /, so that an encoded string can be used safely in a URL. This scheme also uses the padding character =.

The BASE64URLNOPAD encoding uses the same alphabet as BASE6URL, but leaves out the padding. Thus the length of an encoding with this scheme is not necessarily a multiple of four.

The BASE64CF` is similar to ``BASE64URL, with the following changes to BASE64: + replaced with -, / replaced with ~ and _ as the padding character. It is used by a certain CDN provider who also inspired the name.

The case ENUM MUST be set to DEFAULT for for all of the BASE64* encodings.

HEX

The HEX encoding scheme converts hex strings into blobs and vice versa. For encodings, you may use the case ENUM to specify upper- or lowercase hex digits A through f (default DEFAULT, which means the same as LOWER). A prefix such as 0x is not used for an encoding and is illegal for a decoding.

If a hex string to be decoded has an odd number of digits, it is decoded as if a 0 is prepended to it; that is, the first digit is interpreted as representing the least significant nibble of the first byte. For example:

# The concatenated string is "abcdef0", and is decoded as "0abcdef0".
set resp.http.First = "abc";
set resp.http.Second = "def0";
set resp.http.Hex-Decoded


    = blob.encode(HEX, blob=blob.decode(HEX,


                       encoded=resp.http.First + resp.http.Second));

URL

The URL decoding replaces any %<2-hex-digits> substrings with the binary value of the hexadecimal number after the % sign.

The URL encoding implements "percent encoding" as per RFC3986. The case ENUM determines the case of the hex digits, but does not affect alphabetic characters that are not percent-encoded.

BLOB decode(ENUM decoding, INT length, STRING encoded)

BLOB decode(


   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, BASE64CF, HEX, URL} decoding=IDENTITY,


   INT length=0,


   STRING encoded
)

Returns the BLOB derived from the string encoded according to the scheme specified by decoding.

If length > 0, only decode the first length characters of the encoded string. If length <= 0 or greater than the length of the string, then decode the entire string. The default value of length is 0.

decoding defaults to IDENTITY.

Example:

blob.decode(BASE64, encoded="Zm9vYmFyYmF6");
# same with named parameters
blob.decode(encoded="Zm9vYmFyYmF6", decoding=BASE64);
# convert string to blob
blob.decode(encoded="foo");

STRING encode(ENUM encoding, ENUM case, BLOB blob)

STRING encode(


   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, BASE64CF, HEX, URL} encoding=IDENTITY,


   ENUM {LOWER, UPPER, DEFAULT} case=DEFAULT,


   BLOB blob
)

Returns a string representation of the BLOB blob as specified by encoding. case determines the case of hex digits for the HEX and URL encodings, and is ignored for the other encodings.

encoding defaults to IDENTITY, and case defaults to DEFAULT. DEFAULT is interpreted as LOWER for the HEX and URL encodings, and is the required value for the other encodings.

Example:

set resp.http.encode1


    = blob.encode(HEX,


                  blob=blob.decode(BASE64, encoded="Zm9vYmFyYmF6"));
# same with named parameters
set resp.http.encode2


    = blob.encode(blob=blob.decode(encoded="Zm9vYmFyYmF6",


                                           decoding=BASE64),


                      encoding=HEX);
# convert blob to string
set resp.http.encode3


    = blob.encode(blob=blob.decode(encoded="foo"));

STRING transcode(ENUM decoding, ENUM encoding, ENUM case, INT length, STRING encoded)

STRING transcode(


   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, BASE64CF, HEX, URL} decoding=IDENTITY,


   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, BASE64CF, HEX, URL} encoding=IDENTITY,


   ENUM {LOWER, UPPER, DEFAULT} case=DEFAULT,


   INT length=0,


   STRING encoded
)

Translates from one encoding to another, by first decoding the string encoded according to the scheme decoding, and then returning the encoding of the resulting blob according to the scheme encoding. case determines the case of hex digits for the HEX and URL encodings, and is ignored for other encodings.

As with blob.decode(): If length > 0, only decode the first length characters of the encoded string, otherwise decode the entire string. The default value of length is 0.

decoding and encoding default to IDENTITY, and case defaults to DEFAULT. DEFAULT is interpreted as LOWER for the HEX and URL encodings, and is the required value for the other encodings.

Example:

set resp.http.Hex2Base64-1


     = blob.transcode(HEX, BASE64, encoded="666f6f");


 # same with named parameters


 set resp.http.Hex2Base64-2


    = blob.transcode(encoded="666f6f",


                          encoding=BASE64, decoding=HEX);


 # URL decode -- recall that IDENTITY is the default encoding.


 set resp.http.urldecoded


    = blob.transcode(encoded="foo%20bar", decoding=URL);


 # URL encode


 set resp.http.urlencoded


     = blob.transcode(encoded="foo bar", encoding=URL);

BOOL same(BLOB, BLOB)

Returns true if and only if the two BLOB arguments are the same object, i.e. they specify exactly the same region of memory, or both are empty.

If the BLOBs are both empty (length is 0 and/or the internal pointer is NULL), then blob.same() returns true. If any non-empty BLOB is compared to an empty BLOB, then blob.same() returns false.

BOOL equal(BLOB, BLOB)

Returns true if and only if the two BLOB arguments have equal contents (possibly in different memory regions).

As with blob.same(): If the BLOBs are both empty, then blob.equal() returns true. If any non-empty BLOB is compared to an empty BLOB, then blob.equal() returns false.

INT length(BLOB)

Returns the length of the BLOB.

BLOB sub(BLOB, BYTES length, BYTES offset=0)

Returns a new BLOB formed from length bytes of the BLOB argument starting at offset bytes from the start of its memory region. The default value of offset is 0B.

blob.sub() fails and returns NULL if the BLOB argument is empty, or if offset + length requires more bytes than are available in the BLOB.

new xblob = blob.blob(ENUM decoding, STRING encoded)

new xblob = blob.blob(


   ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, BASE64CF, HEX, URL} decoding=IDENTITY,


   STRING encoded
)

Creates an object that contains the BLOB derived from the string encoded according to the scheme decoding.

Example:

new theblob1 = blob.blob(BASE64, encoded="YmxvYg==");
# same with named arguments
new theblob2 = blob.blob(encoded="YmxvYg==", decoding=BASE64);
# string as a blob
new stringblob = blob.blob(encoded="bazz");

BLOB xblob.get()

Returns the BLOB created by the constructor.

Example:

set resp.http.The-Blob1 =


    blob.encode(blob=theblob1.get());
set resp.http.The-Blob2 =


    blob.encode(blob=theblob2.get());
set resp.http.The-Stringblob =


    blob.encode(blob=stringblob.get());

STRING xblob.encode(ENUM encoding, ENUM case)

STRING xblob.encode(


      ENUM {IDENTITY, BASE64, BASE64URL, BASE64URLNOPAD, BASE64CF, HEX, URL} encoding=IDENTITY,


      ENUM {LOWER, UPPER, DEFAULT} case=DEFAULT
)

Returns an encoding of BLOB created by the constructor, according to the scheme encoding. case determines the case of hex digits for the HEX and URL encodings, and MUST be set to DEFAULT for the other encodings.

Example:

# blob as text
set resp.http.The-Blob = theblob1.encode();
# blob as base64
set resp.http.The-Blob-b64 = theblob1.encode(BASE64);

For any blob.blob() object, encoding and case, encodings via the xblob.encode() method and the blob.encode() function are equal:

# Always true:
blob.encode(ENC, CASE, blob.get()) == blob.encode(ENC, CASE)

But the xblob.encode() object method is more efficient -- the encoding is computed once and cached (with allocation in heap memory), and the cached encoding is retrieved on every subsequent call. The blob.encode() function computes the encoding on every call, allocating space for the string in Varnish workspaces.

So if the data in a BLOB are fixed at VCL initialization time, so that its encodings will always be the same, it is better to create a blob.blob() object. The VMOD's functions should be used for data that are not known until runtime.