added sha256sum

sha256sum

@@ -0,0 +1,30 @@
+---
+tags: [ cryptography, hash, checksum, verification ]
+---
+# To compute hash (checksum) of <file>:
+sha256sum <file>
+
+# To compute hash (checksum) from input text:
+# Enter text in keyboard and finish by ctrl-D at beginning of line
+sha256sum
+Some text
+^D
+
+echo -n "Hello World" | sha256sum
+
+# To verify if a hash (checksum) is correct if a hash-file is provided:
+# sha256sum will go thru the files listed in <file-with-list-of-checksums>.
+# For each file it will look if it exists. If not, it will print an error.
+# If it does exist it will compute the checksum and compare it with the hash 
+# provided in the <file-with-list-of-checksums>. If they match (good!) it
+# will print "filename: OK". 
+# If they don't match (warning, you might have been hacked), it will 
+# print "filename: FAILED".
+sha256sum --check <file-with-list-of-checksums>
+
+# Same as above (but ignore missing files):
+# This tells you if your file is an exact copy (OK) or has been 
+# tampered with (FAILED) without bothering you with warnings about
+# missing files
+sha256sum --check --ignore-missing <file-with-list-of-checksums>
+