What is AAAAIGZ0eXBpc29t?

If you work with Base64-encoded videos, you have seen strings like AAAAIGZ0eXBpc29t, AAAAIGZ0eXBtcDQy or AAAAHGZ0eXBNNEEg many times. They are the Base64 form of the first bytes of an MP4 (ISO base media) file.

Decoding the prefix

AAAAIGZ0eXBpc29t decodes to these 12 bytes:

00 00 00 20  66 74 79 70  69 73 6F 6D
└─ size ──┘  └ "ftyp" ──┘  └ "isom" ─┘

• 00 00 00 20 = 32: the size of the first box (MP4 files are a sequence of boxes).
• ftyp: the box type, “file type”.
• isom: the major brand, meaning a generic ISO base media file.

Other brands and box sizes produce close relatives:

The ftyp marker starts at byte 4, which is not a multiple of 3, so it is not encoded on its own. But because box sizes are multiples of 4, the fragment GZ0eXB appears at the same place in all of them. If a Base64 string starts with AAAA and contains GZ0eXB at position 5, it is an MP4-family file.

Every format has a fingerprint

Files start with a fixed “magic number”, and when it is aligned to a 3-byte boundary its Base64 form is fixed too:

See more on our popular Base64 strings page, or let the Base64 to File decoder identify the file for you — it recognises more than 50 formats from their signatures, no matter what file name or MIME type you were given.