Generating v5 UUID. What is name and namespace?

Question

I've read the man page, but I do not understand what name and namespace are for.

For version 3 and version 5 UUIDs the additional command line arguments namespace and name have to be given. The namespace is either a UUID in string representation or an identifier for internally pre-defined namespace UUIDs (currently known are "ns:DNS", "ns:URL", "ns:OID", and "ns:X500"). The name is a string of arbitrary length.

The namespace:

The namespace is either a UUID in string representation or an

Does it mean that I need to store it (UUID v4) somewhere in relation to the generated UUID v5? In either case, why is this not done automatically?

The name is a string of arbitrary length.

name a completely random string? What is the purpose of it then? Can it be decoded from the UUID v5?

Answer 1

Type 3 and Type 5 UUIDs are just a technique of stuffing a hash into a UUID:

• Type 1: stuffs MAC address+datetime into 128 bits
• Type 3: stuffs an MD5 hash into 128 bits
• Type 4: stuffs random data into 128 bits
• Type 5: stuffs an SHA1 hash into 128 bits
• Type 6: unofficial idea for sequential UUIDs

Edit: Unofficial type 6 now has an official rfc

An SHA1 hash outputs 160 bits (20 bytes); the result of the hash is converted into a UUID.

With a 20-byte digest from SHA1:

SHA1 Digest:   74738ff5 5367 e958 1aee 98fffdcd1876 94028007
UUID (v5):     74738ff5-5367-5958-9aee-98fffdcd1876
                             ?    ?first two bits set to 1 and 0, respectively
                             ╰─low nibble is set to 5, to indicate type 5

What do I hash?

You're probably wondering what is it that I'm supposed to hash. Basically you hash the concatenation of:

sha1(NamespaceUUID+AnyString);

You prefix your string with a so-called namespace to prevent name conflicts.

The UUID RFC pre-defines four namespaces for you:

• NameSpace_DNS: {6ba7b810-9dad-11d1-80b4-00c04fd430c8}
• NameSpace_URL: {6ba7b811-9dad-11d1-80b4-00c04fd430c8}
• NameSpace_OID: {6ba7b812-9dad-11d1-80b4-00c04fd430c8}
• NameSpace_X500:{6ba7b814-9dad-11d1-80b4-00c04fd430c8}

So, you could hash together:

StackOverflowDnsUUID = sha1(Namespace_DNS + "stackoverflow.com");
StackOverflowUrlUUID = sha1(Namespace_URL + "stackoverflow.com");

The RFC then defines how to:

• take the 160 bits from SHA1
• and convert it into 128 bits of a UUID

The basic gist is to only take the first 128 bits, stuff a 5 in the type record, and then set the first two bits of the clock_seq_hi_and_reserved section to 1 and 0, respectively.

More examples

Now that you have a function that generates a so-called Name , you can have the function (in pseudo-code):

UUID NameToUUID(UUID NamespaceUUID, String Name)
{
    //Note: All code on stackoverflow is public domain - no attribution required.

    Byte[] hash = sha1(NamespaceUUID.ToBytes() + Name.ToBytes());
    Uuid result;

    //Copy first 16-bytes of the hash into our Uuid result
    Copy(hash, result, 16);

    //set high-nibble to 5 to indicate type 5
    result[6] &= 0x0F; 
    result[6] |= 0x50; 

    //set upper two bits to "10"
    result[8] &= 0x3F; 
    result[8] |= 0x80; 

    return result;
}

(Note: the endian-ness of your system can affect indices of the above bytes)

Now you can have calls:

uuid = NameToUUID(Namespace_DNS, 'www.stackoverflow.com');
uuid = NameToUUID(Namespace_DNS, 'www.google.com');
uuid = NameToUUID(Namespace_URL, 'http://www.stackoverflow.com');
uuid = NameToUUID(Namespace_URL, 'http://www.google.com/search&q=rfc+4112');
uuid = NameToUUID(Namespace_URL, 'http://stackoverflow.com/questions/5515880/test-vectors-for-uuid-version-5-converting-hash-into-guid-algorithm');

Now back to your question

For version 3 and version 5 UUIDs the additional command line arguments namespace and name have to be given. The namespace is either a UUID in string representation or an identifier for internally pre-defined namespace UUIDs (currently known are "ns:DNS", "ns:URL", "ns:OID", and "ns:X500"). The name is a string of arbitrary length.

The namespace is whatever UUID you like. It can be one of the pre-defined ones, or you can make up your own, e.g.¹:

UUID Namespace_RectalForeignExtractedObject = '8e884ace-bee4-11e4-8dfc-aa07a5b093db'

The name is a string of arbitrary length.

The name is just the text you want to have appended to the namespace, then hashed, and stuffed into a UUID:

uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'screwdriver');
uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'toothbrush');
uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'broomstick');
uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'orange');
uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'axe handle');
uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'impulse body spray');
uuid = NameToUUID('8e884ace-bee4-11e4-8dfc-aa07a5b093db', 'iPod Touch');