sha256-final

Arguments: context &key return

Finalize and return the SHA256 hash from context in a format determined by the return keyword argument.

return can be one of the following values:

• :integer (the default): the return value is an integer of up to 128 bits representing the digest. Warning: note that an integer representation of the digest potentially loses information if the generated signature has leading octets containing only zeroes. Care should be taking when using this return type if you intend to further encode the signature (such as to a base64-encoded string). See the example below.
• :usb8: return an (unsigned-byte 8) array containing the digest.
• :hex: return a string containing the hexadecimal representation of the digest. See hex-string-to-integer.

cl-user(4): (setq c (sha256-init))
#S(excl::md-context :ctx 9183616 :size 32)
cl-user(5): (sha256-update c "foo")
cl-user(6): (sha256-final c :return :usb8)
#(44 38 180 107 104 255 198 143 249 155 ...)

;;  Potential problem with :return :integer (or :return unspecified
;;  as the default is :integer). The call is to SHA256-FILE which calls
;;  SHA256-FINAL and returns the value SHA256-FINAL returns.
;;
;;  Here we get the sha256 value in a shell:
% openssl dgst -sha256 -hex tab-order.jpg 
SHA256(tab-order.jpg)= 079331d027a95cc63d1466fdf859e0c6b83532199e1c935f30a658bf8e5f8e29

;; Now we get it with SHA256-FILE:
cl-user(17): (sha256-file "~/tab-order.jpg")
3426260256022037745305111919104592478490763621909092613016648559449698438697
cl-user(18): (format t "~x" *)
79331d027a95cc63d1466fdf859e0c6b83532199e1c935f30a658bf8e5f8e29
nil
cl-user(19): 

;; Note the hex values are the same but the leading 0 is not printed
;; by Lisp, which is dealing with the value as an integer. The leading
;; 0 is preserved when the :return is :hex:
cl-user(21): (sha256-file "~/tab-order.jpg" :return :hex)
"079331d027a95cc63d1466fdf859e0c6b83532199e1c935f30a658bf8e5f8e29"
cl-user(22): 

See MD*, SHA*, HMAC, and other message digest support in miscellaneous.htm. See also digest-final.

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This page was not revised from the 10.0 page.
Created 2019.8.20.