CODEWITHSUNDEEP
python.netuuid
MidnightThoughtful
MidnightThoughtful

Reputation: 233

Emulate encoding UUID to Base64

I am attempting to emulate the way a C# application converts a UUID to a Base64 value. For some reason, I can get part of the string to match the expected value, but not the whole string.

The C# code I was given:

public static string ToShortGuid(this Guid newGuid) {
string modifiedBase64 = Convert.ToBase64String(newGuid.ToByteArray())
.Replace('+', '-').Replace('/', '_') // avoid invalid URL characters
.Substring(0, 22);
return modifiedBase64;
}

What I have tried in Python 3.6:

import uuid
import base64

encode_str = = base64.urlsafe_b64encode(uuid.UUID("fa190535-6b00-4452-8ab1-319c73082b60").bytes)
print(encode_str)

"fa190535-6b00-4452-8ab1-319c73082b60" is a known UUID, and the application apparently uses the above c# code to generate a 'ShortGuid' value of "NQUZ-gBrUkSKsTGccwgrYA".

When I process the same UUID through my Python code, I get: "-hkFNWsARFKKsTGccwgrYA=="

From both of these output strings, this part matches: "KsTGccwgrYA", but the rest doesn't.

Upvotes: 3

Views: 4372

Answers (2)

melpomene
melpomene

Reputation: 85767

NQUZ-gBrUkSKsTGccwgrYA corresponds to a byte sequence of 350519fa006b52448ab1319c73082b60.

If we add - in the appropriate locations, we get:

 350519fa-006b-5244-8ab1-319c73082b60
#   \/     \/   \/
#   /\     /\   /\
 fa190535-6b00-4452-8ab1-319c73082b60

Compared to the known UUID you started with, the bytes are the same, but the order within the first 3 subgroups is reversed.

To emulate what .NET does, you need to use UUID.bytes_le:

The UUID as a 16-byte string (with time_low, time_mid, and time_hi_version in little-endian byte order).

See also Why does Guid.ToByteArray() order the bytes the way it does?

Upvotes: 8

John Zwinck
John Zwinck

Reputation: 249293

You need to use bytes_le to get the endianness to match Microsoft's:

base64.urlsafe_b64encode(uuid.UUID("fa190535-6b00-4452-8ab1-319c73082b60").bytes_le)

That gives b'NQUZ-gBrUkSKsTGccwgrYA=='.

Upvotes: 6

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