<s>
The	O
Needham	B-Protocol
–	I-Protocol
Schroeder	I-Protocol
protocol	I-Protocol
is	O
one	O
of	O
the	O
two	O
key	O
transport	O
protocols	O
intended	O
for	O
use	O
over	O
an	O
insecure	O
network	O
,	O
both	O
proposed	O
by	O
Roger	O
Needham	O
and	O
Michael	O
Schroeder	O
.	O
</s>
<s>
The	O
Needham	B-Protocol
–	I-Protocol
Schroeder	I-Protocol
Symmetric	B-Algorithm
Key	I-Algorithm
Protocol	O
,	O
based	O
on	O
a	O
symmetric	B-Algorithm
encryption	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
It	O
forms	O
the	O
basis	O
for	O
the	O
Kerberos	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
The	O
Needham	B-Protocol
–	I-Protocol
Schroeder	I-Protocol
Public-Key	B-Application
Protocol	O
,	O
based	O
on	O
public-key	B-Application
cryptography	I-Application
.	O
</s>
<s>
The	O
protocol	O
can	O
be	O
specified	O
as	O
follows	O
in	O
security	B-General_Concept
protocol	I-General_Concept
notation	I-General_Concept
:	O
</s>
<s>
This	O
flaw	O
is	O
fixed	O
in	O
the	O
Kerberos	B-Protocol
protocol	I-Protocol
by	O
the	O
inclusion	O
of	O
a	O
timestamp	O
.	O
</s>
<s>
This	O
assumes	O
the	O
use	O
of	O
a	O
public-key	B-Application
encryption	I-Application
algorithm	I-Application
.	O
</s>
<s>
Here	O
,	O
Alice	O
and	O
Bob	O
use	O
a	O
trusted	O
server	O
to	O
distribute	O
public	B-Application
keys	I-Application
on	O
request	O
.	O
</s>
<s>
responds	O
with	O
public	B-Application
key	I-Application
alongside	O
'	O
s	O
identity	O
,	O
signed	O
by	O
the	O
server	O
for	O
authentication	O
purposes	O
.	O
</s>
<s>
now	O
knows	O
A	O
wants	O
to	O
communicate	O
,	O
so	O
requests	O
'	O
s	O
public	B-Application
keys	I-Application
.	O
</s>
<s>
As	O
a	O
consequence	O
A	O
uses	O
the	O
public	B-Application
key	I-Application
of	O
I	O
instead	O
of	O
using	O
the	O
public	B-Application
key	I-Application
of	O
B	O
to	O
encrypt	O
the	O
messages	O
she	O
intends	O
to	O
send	O
to	O
her	O
bank	O
.	O
</s>
<s>
Therefore	O
,	O
A	O
sends	O
I	O
her	O
nonce	O
encrypted	O
with	O
the	O
public	B-Application
key	I-Application
of	O
I	O
.	O
I	O
decrypts	O
the	O
message	O
using	O
their	O
private	B-Application
key	I-Application
and	O
contacts	O
B	O
sending	O
it	O
the	O
nonce	O
of	O
A	O
encrypted	O
with	O
the	O
public	B-Application
key	I-Application
of	O
B	O
.	O
</s>
<s>
B	O
responds	O
with	O
their	O
own	O
nonce	O
and	O
encrypts	O
the	O
message	O
with	O
the	O
public	B-Application
key	I-Application
of	O
A	O
.	O
</s>
<s>
Since	O
I	O
is	O
not	O
in	O
possession	O
of	O
the	O
private	B-Application
key	I-Application
of	O
A	O
they	O
have	O
to	O
relay	O
the	O
message	O
to	O
A	O
without	O
knowing	O
the	O
content	O
.	O
</s>
<s>
A	O
decrypts	O
the	O
message	O
with	O
her	O
private	B-Application
key	I-Application
and	O
respond	O
with	O
the	O
nonce	O
of	O
B	O
encrypted	O
with	O
the	O
public	B-Application
key	I-Application
of	O
I	O
.	O
I	O
decrypts	O
the	O
message	O
using	O
their	O
private	B-Application
key	I-Application
and	O
is	O
now	O
in	O
possession	O
of	O
nonce	O
A	O
and	O
B	O
.	O
</s>
<s>
The	O
paper	O
also	O
describes	O
a	O
fixed	O
version	O
of	O
the	O
scheme	O
,	O
referred	O
to	O
as	O
the	O
Needham	B-Protocol
–	I-Protocol
Schroeder	I-Protocol
–	O
Lowe	O
protocol	O
.	O
</s>
