问题
Maybe a completely stupid question but I just cannot work it out...
First I need to generate an SHA-1 hash using part of my submission markup. The hash is correct and the output is;
0623f7917a1e2e09e7bcc700482392fba620e6a2
Next I need to base64 encode this hash to a 28 character sting. This is where I am struggling as when I run my code (or use the online generators) I get a 56 character sting. The sting I get is;
MDYyM2Y3OTE3YTFlMmUwOWU3YmNjNzAwNDgyMzkyZmJhNjIwZTZhMg==
Question is 1) Is it possible to get a 28 char string from the hash above? and 2) how... where could I be going wrong.
Thank you for any help provided.
回答1:
A SHA-1 hash is 20 bytes long, but those bytes are unlikely to all be printable characters. Hence if we want to display those 20 bytes to a human we have to encode them in printable characters.
One way to do this is hexadecimal, where we take each byte, chop it in half and represent each half (a 4-bit value, numerically 0-15) with characters in the range 0123456789abcdef.
Thus each byte is encoded into 2 hex values, so our 20-byte hash value is encoded in 40 bytes of printable characters.
Hex is simple to calculate and it's easy for a human to look at an encoding and work out what the bytes actually look like, but it's not the most efficient as we're only using 16 out of the 95 ASCII printable characters.
Another way to encode arbitrary binary data into printable characters is Base 64. This is more efficient, encoding (on average) 3 bytes in 4 base64 values, but it's a lot harder for a human to parse the encoding.
The behaviour you are seeing is due to encoding a 20-byte hash value into 40 bytes of hex, and then encoding those 40 bytes of hex into 56 bytes (40 / 3 * 4, then rounded up to the nearest 4 bytes) of base64 data.
You need to either encode directly to base64 from the raw hash bytes (if available), or decode the hexadecimal value to bytes before encoding to base64.
来源：https://stackoverflow.com/questions/20075378/why-does-my-base64-encoded-sha-1-hash-contain-56-chars