A method, comprising: selecting at least two verification code generators from a verification code generator set comprising a plurality of verification code generators to compose a current use set, wherein the at least two verification code generators include a first verification code generator and a second verification code generator, and the first verification code generator and the second verification code generator are associated with different verification code generators, and wherein the first verification code generator and the second verification code generator each includes a numerical verification code generator, a Chinese verification code generator, a picture-based verification code generator, an ASCII character verification code generator, or a question-and-answer verification code generator, wherein the selecting of the at least two verification code generators from a verification code generator set comprises: determining a number of verification code generators for the at least two verification code generators based on a high risk setting or a low risk setting, wherein the high risk setting relates to a number of login failures within a fixed time segment being equal to or exceeding a predetermined login failure threshold, wherein the low risk setting relates to a number of login failures within the fixed time segment failing to equal to or exceed the predetermined login failure threshold, and wherein the number of verification code generators associated with the high risk setting is greater than the number of verification code generators associated with the low risk setting; executing each verification code generator in the current use set to obtain corresponding partial verification codes; composing a current verification code from the partial verification codes; outputting the current verification code to a user via a client; receiving a user input sent back by the client that is made in response to the current verification code; and comparing the current verification code and the user input sent back by the client to determine whether the user is verified. selecting at least two verification code generators from a verification code generator set comprising a plurality of verification code generators to compose a current use set, wherein the at least two verification code generators include a first verification code generator and a second verification code generator, and the first verification code generator and the second verification code generator are associated with different verification code generators, and wherein the first verification code generator and the second verification code generator each includes a numerical verification code generator, a Chinese verification code generator, a picture-based verification code generator, an ASCII character verification code generator, or a question-and-answer verification code generator, wherein the selecting of the at least two verification code generators from a verification code generator set comprises: determining a number of verification code generators for the at least two verification code generators based on a high risk setting or a low risk setting, wherein the high risk setting relates to a number of login failures within a fixed time segment being equal to or exceeding a predetermined login failure threshold, wherein the low risk setting relates to a number of login failures within the fixed time segment failing to equal to or exceed the predetermined login failure threshold, and wherein the number of verification code generators associated with the high risk setting is greater than the number of verification code generators associated with the low risk setting; executing each verification code generator in the current use set to obtain corresponding partial verification codes; composing a current verification code from the partial verification codes; outputting the current verification code to a user via a client; receiving a user input sent back by the client that is made in response to the current verification code; and comparing the current verification code and the user input sent back by the client to determine whether the user is verified.