How to structure DKIM headers - email

Question

I'm working on a system that sends emails from the ground up.
I'm currently working on a DKIM signer, I know how to do it, but cannot find how to format the header and body before hashing them.
Does anyone know this? Already thanks.

Answer 1

3.4.1. The "simple" Header Canonicalization Algorithm
The "simple" header canonicalization algorithm does not change header
fields in any way. Header fields MUST be presented to the signing or
verification algorithm exactly as they are in the message being
signed or verified. In particular, header field names MUST NOT be
case folded and whitespace MUST NOT be changed.
3.4.2. The "relaxed" Header Canonicalization Algorithm
The "relaxed" header canonicalization algorithm MUST apply the
following steps in order:
Convert all header field names (not the header field values) to
lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
Unfold all header field continuation lines as described in
[RFC5322]; in particular, lines with terminators embedded in
continued header field values (that is, CRLF sequences followed by
WSP) MUST be interpreted without the CRLF. Implementations MUST
NOT remove the CRLF at the end of the header field value.
Convert all sequences of one or more WSP characters to a single SP
character. WSP characters here include those before and after a
line folding boundary.
Delete all WSP characters at the end of each unfolded header field
value.
Delete any WSP characters remaining before and after the colon
separating the header field name from the header field value. The
colon separator MUST be retained.
3.4.3. The "simple" Body Canonicalization Algorithm
The "simple" body canonicalization algorithm ignores all empty lines
at the end of the message body. An empty line is a line of zero
length after removal of the line terminator. If there is no body or
no trailing CRLF on the message body, a CRLF is added. It makes no
other changes to the message body. In more formal terms, the
"simple" body canonicalization algorithm converts "*CRLF" at the end
of the body to a single "CRLF".
Note that a completely empty or missing body is canonicalized as a
single "CRLF"; that is, the canonicalized length will be 2 octets.
The SHA-1 value (in base64) for an empty body (canonicalized to a
"CRLF") is:
uoq1oCgLlTqpdDX/iUbLy7J1Wic=
The SHA-256 value is:
frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY=
3.4.4. The "relaxed" Body Canonicalization Algorithm
The "relaxed" body canonicalization algorithm MUST apply the
following steps (1) and (2) in order:
Reduce whitespace:
Ignore all whitespace at the end of lines. Implementations
MUST NOT remove the CRLF at the end of the line.
Reduce all sequences of WSP within a line to a single SP
character.
Ignore all empty lines at the end of the message body. "Empty
line" is defined in Section 3.4.3. If the body is non-empty but
does not end with a CRLF, a CRLF is added. (For email, this is
only possible when using extensions to SMTP or non-SMTP transport
mechanisms.)
The SHA-1 value (in base64) for an empty body (canonicalized to a
null input) is:
2jmj7l5rSw0yVb/vlWAYkK/YBwk=
The SHA-256 value is:
47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
3.4.5. Canonicalization Examples (INFORMATIVE)
In the following examples, actual whitespace is used only for
clarity. The actual input and output text is designated using
bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a
tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
For example, "X <SP> Y" and "X<SP>Y" represent the same three
characters.
Example 1: A message reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF>
<CRLF>
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>
<CRLF>
<CRLF>
when canonicalized using relaxed canonicalization for both header and
body results in a header reading:
a:X <CRLF>
b:Y <SP> Z <CRLF>
and a body reading:
<SP> C <CRLF>
D <SP> E <CRLF>
Example 2: The same message canonicalized using simple
canonicalization for both header and body results in a header
reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF>
and a body reading:
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>
Example 3: When processed using relaxed header canonicalization and
simple body canonicalization, the canonicalized version has a header
of:
a:X <CRLF>
b:Y <SP> Z <CRLF>
and a body reading:
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>